YOU'VE HEARD the expression "Sex, drugs, rock 'n' roll and diapers"? No? There may be a reason for that.

The uneasy juxtaposition of parenthood and the rock lifestyle isn't news to vocalist Lisa Mathews and guitarist Mikel Gehl. It was having kids that inspired the friends, who spent years performing in the popular Baltimore alt-rock band Love Riot, to try their hands at making music for children. "Immediately I said, 'Having a baby takes so much time and so much attention, there is no way I ever want to go into a smoky bar and come home at 4 in the morning and try to be a good mother,' " Mathews recalls. "It just didn't seem to work in the new equation of family," Gehl agrees.

Together, they found something that did: Milkshake, a children's rock band.

A recent weekday afternoon found Mathews, Gehl and a full band -- they bill themselves as "an expandable duo" -- performing in XM Satellite Radio's sleek recording studio. At the invitation of children's programming director Kenny Curtis, an early supporter of Milkshake, the band is recording songs for an upcoming Thanksgiving special. "He's the first guy who paid attention to us," Mathews says. "We weren't sure what we were doing, we just knew that we wrote these songs for our kids and we recorded them. And then people started really liking it, and Kenny listened to it and he e-mailed and said, 'This is the best thing I've heard in 13 years!' "

The disc that prompted Curtis's enthusiasm was Milkshake's ebullient 2002 debut, "Happy Songs." The band's second release, "Bottle of Sunshine," a high-energy pastiche of musical styles epitomized by its infectious title track, came out last month. In celebration, Milkshake plays a CD release party Sunday afternoon at Arlington's Iota Club & Cafe.

Mathews and Gehl, both in their forties, acknowledge that this happy outcome was not a foregone conclusion. After the birth of her daughter, Mathews says, "I didn't even want to do music, but Mikel said, 'No, come on,' and came over with his guitar." At the time, Love Riot was on hiatus and the pair were writing soundtrack music for television -- from "One Life to Live" to "Homicide: Life on the Street" -- but missed performing live. (Hey, who wouldn't, if they'd opened for Men Without Hats?)When Gehl's wife, Donna, became pregnant about a year later, Gehl and Mathews began work on a collection of children's songs. Mathews's daughter, Jesse, is now 4; Gehl's son Eric nearly 3. "We actually started writing a lot of the songs when Eric was on the way," Gehl says. "I have nieces and nephews that I hang out with constantly, so I was familiar with the subject material. And the kid experience was just so different for us, it just felt like where we should be."

The new venture, which provided both a baby-friendly lifestyle and a vehicle for the new emotions prompted by parenthood, was a learn-as-you-go affair. "At first, with 'Happy Songs,' I don't think we really knew what we were gonna do," Mathews says. "We just started doing it." They did have some basic precepts in mind, however. "We wanted to inspire them to learn and have fun at the same time," Gehl says. "And not drive the parents crazy," Mathews hastily adds. "Because kids like to hear things over and over and over." Gehl says it clicked from the start. "As soon as we started doing it, it just felt like, 'This is exactly what we should be doing,' " he recalls. "It wasn't at all planned out, we could've only fallen into it."

Other rockers-turned-children's-artists say dissatisfaction with existing kids' music prompted them to make their own. Mathews, though, says her lack of familiarity with the genre was an asset. "I personally didn't really play quote unquote 'children's music' for my girl when she was a baby. I found myself gravitating toward world music instead. I think because I didn't really listen to other children's music per se -- Raffi and the other major children's artists out there. Because of that, we didn't try to write children's music, we just wrote music . . . "

" . . . that included children," says Gehl, finishing her sentence.

Much of Milkshake's material is written from a child's perspective. "I don't see how anyone who does not have a child can write music for kids," Mathews says. "I know it's done, but . . . . I get so many wonderful ideas from Jesse." What happens when they grow up? "I think our music will evolve as our children grow older," Mathews says. "And then we'll just stop doing it when they're 10 and be a rock band again."

Not that they ever stopped -- although the band's songs are about things like eating breakfast and going to school, Milkshake's music is edgier than much children's fare. "We still retained our rock ties, our rock sensibility," Mathews says. "We've been doing it for so long together that it's kind of hard to lose the quote unquote 'Love Riot sound,' or at least the rock band sound."

Mathews and Gehl have played together in various musical incarnations since 1986, when Gehl's band ran an ad for a singer in the Village Voice. The band was Beyond Words, a precursor to Love Riot. A lot has changed since then. "It's funny," Mathews says, "Mikel and I, when we were in the rock band, we were always thinking about what to wear and how many sets we were going to play, and now. . . . " Gehl jumps in, "Now it's what props to make for the kids!"

Both musicians say the learning curve has been steep. Performing for kids, they say, far from taking the pressure off, turns it up. Way up. "It's a challenge to do a great show," Mathews says. "We used to be, 'Here's the set and you don't have to waver from the set unless you break a guitar string.' We didn't have to worry about entertaining as much if the audience just came to hear music. Whereas kids -- oh my! -- what a difference. We have an idea of a set, but it changes depending on the age of the kids." And, Gehl adds, "what they appear to be in the mood for."

"You gotta keep it going," Mathews says.

One way to do that is with visuals. This explains the brightly colored tutus that Mathews wears for live performances and props such as the "bottle of sunshine" that accompanies the song of the same name. "It's literally a bottle," Mathews says. "I pour it on the kids and they scream with delight, yellow confetti coming down on them." Gehl says the band talked to teachers and librarians for guidance on getting ideas across to kids. "We spoke to them about how to phrase certain things and form ideas in a way that would be easier for kids to understand." He says he's amazed at how much his young audiences take in. "I think they get the metaphors," Gehl continues. " 'Bottle of Sunshine' is about cheering yourself up -- and they get that."

The best moments, though, are more spontaneous than cerebral. "This is a ton of work, very much like any professional thing that you do," Gehl says. "But it's funny, when we're with the kids, it's nothing but fun." Mathews agrees. "We're having the best time," she says. "I think even if we didn't make money at it, I would still do it."

As for Love Riot, its future is undecided. For now, Mathews and Gehl are too busy with Milkshake to give it much thought. "It immediately got so busy we figured it was a good sign," Gehl says. "We're working more than we ever did with Love Riot -- and Love Riot was fairly successful," Mathews says. "And it's a whole different environment; you can play schools and museums and libraries and festivals, even some rock clubs. It's kinda neat to come full circle and do the Iota show."

In some cases, Love Riot fans have come full circle along with the band. "What's fun now is that a lot of Love Riot fans have kids and they're like, 'Oooooh!' especially for full band shows. They seem so happy, and we're so happy -- it's a beautiful creation."

"Parenting is such a community thing anyway," Gehl says. "It's like we're all in it together."

MILKSHAKE CD RELEASE PARTY -- Sunday at 3 at Iota, 2832 Wilson Blvd., Arlington. $10, children $6. 703-522-8340.

Milkshake -- Mikel Gehl and Lisa Mathews -- will get youngsters shaking at the duo's CD release party at Iota for "Bottle of Sunshine." The kid rockers are former members of the group Love Riot.