Braves third baseman Martin Prado beats Bryce Harper to the bag as the Nats rookie goes for too much with an attempted triple. (Toni L. Sandys/The Washington Post)

After the Washington Nationals lost, 3-2, to the Braves on Sunday, hundreds of children were allowed on the field to run the bases. They sprinted, low-fived the Teddy Roosevelt mascot and loved every second. You could tell they were Bryce Harper fans, too. Not one of them stopped at second base.

The Nationals play with the exuberance of gifted children. But they also make the mistakes of youth. In this game, Harper turned an 0-2 count into a clutch late-inning walk, drilled a 390-foot double and homered to the second deck. But he also got thrown out trying to stretch for a triple with only one out and the heart of the Nats’ order due up.

That was just one of a half-dozen crucial Nats mistakes in fundamentals — such asGio Gonzalez being a step slow to cover first base — that turned a likely win into a jaw-grinding unnecessary loss. “My ears are deaf from hearing [pitching coach Steve] McCatty yelling, ‘Get over,’ ” said Manager Davey Johnson of the inning-opening lapse by Gonzalez that ignited Atlanta’s two-run game-winning rally.

When you’re in first place, as the Nats still are, baseball changes. Gaps in ability shrink. You are your habits, perhaps even more than your talents.

This glistening afternoon before 38,046 shouldn’t have produced a loss. Steve Lombardozzi and Harper became the first rookies to hit back-to-back home runs to lead off a game in the modern major league era, since 1900, according to Elias Sports Bureau. And on consecutive pitches, too. Fully formed teams don’t get shut out for nine innings after such a start. But mature is what the Nats want to become, not what they are.

“The ingredients are there,” said Johnson. “We’re not a unit yet.”

Coming after Stephen Strasburg’s hint-of-Koufax game on Saturday, the Nats seemed poised for a two-game sweep behind Gonzalez, who leads MLB in strikeouts. But poised is what they weren’t.

Early in the year, the Nats often got away with mistakes. Now they’re in the midst of playing the best of the AL East and NL East — all teams with winning records — for 32 games in a row. So far they’re 7-7, but face six more series against teams that take advantage of minor mistakes.

For two days, the reunited heart of the Nats order, Ryan Zimmerman, Adam LaRoche and rusty Mike Morse, has been overanxious and unproductive. Just when there are more shoulders to lift, they grunt too hard.

Even when Atlanta tried to be helpful, walking the first two hitters in the eighth inning, Zimmerman, a graybeard of 26, ignored the first tenet of situational hitting: Think — what is the situation?

“Zim was over-aggressive early in the count. Drive it out of the park,” said Johnson, “instead of using the whole field, like he’s usually so good at.”

Instead of an opposite field hit that might have tied the game and moved Harper to third base, Zimmerman fell behind 0-2 and grounded into his eighth double play. Zimmerman just stinks right now. Actually, those were his exact postgame words and not long after showering, too.

The Nats admit their mistakes but repeat them too much, nonetheless.

“My fault,” said Gonzalez. “I should have gone out there and exploded right off the mound and got that ball. Was lost a little bit in the stands but it’s no excuse.”

That’s why the expression, “Be on guard” was invented.

Just two months ago, nagging the so-recently-awful Nats about details of the game might’ve seemed cruel. But this is no longer a team trying to post a winning season. Three things have gone extremely right and changed expectations quickly.

First, Gonzalez, despite this poor start, is just one click of focus away from being as good as any starter in baseball. After one-third of his ’12 starts, he’s on pace to go 21-6 with 252 strikeouts in 199 innings, a 2.31 ERA and a microscopic 120 hits allowed. His stuff is close to Strasburg’s. As a one-two punch, they might dominate any postseason series, should the Nats get there. So, the value of an accelerated Nat learning curve has grown.

Second, the Nats have apparently fixed a flaw in Edwin Jackson’s delivery that led him to tip his pitches when he was in his windup. So, he was one of the few pitchers who were better with men on base. Now, he’s normal — and spectacular. In his career, his OPS-against was .787 with none on and .744 with men on base. This year, he’s .592 with nobody on base and .783 with men on.

“We thought we could target some things and help him be an improved version,” said General Manager Mike Rizzo. “Some was mechanical, some psychological and he really trusts his defense now. But it’s not an accident.”

It’s early, but Jackson, the 61-63 career pitcher, may be gone. His 3.17 ERA and improved control may indicate that the Nats have a fourth top-flight starter, not a .500 innings eater.

Finally, Harper, especially hitting second, has raised the Nats’ offense an entire level. The team’s OPS since he was called up is more than 100 points higher than it was before he arrived. Harper, slugging .542 with a .380 on-base percentage, is already so respected that he sees a lower percentage of fastballs than any player in baseball.

“Cool stat,” said Harper.

Also, the 19-year-old wears his mistakes but seems unshaken by them. “They got me. I should have backed off,” he said for being gunned down.

In the outfield and on the bases, Harper can be inspired or blow up. At the plate, he’s also a lit fuse, but of a different kind, one headed toward the dynamite. “I like to guess,” Harper said, “so I can look bad on a pitch early in the count.” Or, if he’s right, a cutter can end in the upper deck off Tommy Hanson or a curve get smacked off the wall for a triple off Roy Halladay.

“That’s why they give you three [strikes]. When you get to two strikes, then spread out [your stance] and battle, battle, battle.”

They’re young. They’re learning. And they’re going to end up being very good. How soon? Depends on the little things, the details that require focus and judgment, the things you absolutely need against the good teams.

How fast will the still-first-place Nats learn? With their schedule, we won’t have to wait long for more answers.

For Thomas Boswell’s previous columns, go to