The best part was the hot dog," our 3-year-old daughter Susan said after Sunday lunch at Hot Diggity Dog." And the ketchup, and the sauerkraut, and the apple juice, the French fries and the cake."

The restaurant, which is in a recycled gas station at 28th and M Streets NW, might not head our list of where to eat chicly for outings with children - a place where everyone can enjoy hot dogs, and not feel guilty about it(they're nitrite-free).

The service was good and fairly quick. There was a lower-priced children's meal, and the staff is relaxed enough for children to do a little walking around while adults finish their coffee.

Susan and her brother Dan, 18 months old, shared a "Puppy Dog." For $1.35 they were served a plain hot dog and what was described as a half order of "American fries." the fries looked like thick potato chips, tasted great, and there were plenty to go around with samples for three adults, too.

Dan, who loves bread and sometimes eats little else in restaurants, especially enjoyed the soft rolls with poppy seeds. He even liked the hot dog and potatoes. The ketchup came in little squeeze packets that our daughter like using with a minimum of mess.

Altogether the hot dogs are fixed in 10 different ways - many of them elaborate. There also are three different toppings for hamburgers, a steak sandwich, homemade chili and a "tunadog." The hot dogs are all-beef, and kosher (so are the buns), and contain no artificial preservatives. They are smaller than the standard variety and brown in color but they taste good. The fries are cooked in peanut oil.

I ordered a "Julia Child" - raw mushrooms, Dijon mustard, onions, Swiss cheese and tomato baked on a hot dog for $1.75. It was pretty good even though the cheese wasn't melted enough and there were too many raw onions for my taste. Our friend ordered the Rueben dog - Russian dressing, sauerkraut and Swiss cheese baked on a hot dog, also $1.75. He liked it.

We also ordered a tossed salad-greens and sliced mushrooms with a piece of pink tomato-that was topped with a delicious yougurt Russian dressing. There was plenty, and sharing is no problem at his restaurant. The waiter was very nice about bringing us extra plates, cups and utensils. A side order of sauerkraut was good and hot, and we all had some.

My husband ordered a mushroom cheeseburger for $2.45. It was large and good, but again had too much raw onion. He also orded a glass of Purpom hard cider from France. It was not very hard, and the glass, alas, was small. There is a good selection of wines and beers - some of them exotic.

For children not only are the usual soft drinks available, but also three fruit juices - apple, grapefruit and organe - plus V-8. Sue and Dan gulped down the apple juice as quickly as they do at home.

The best part of the meal, however, was dessert - apple pan dowdy, which the restaurant says is made fresh daily and topped with real, rich whipped cream. The price is rather steep, $1.50, but it's better than this mother ever made.

The restaurant has tables indoors and out, and most of the latter shaded by pear trees. We sat outside so the children could move around a little without disturbing others. A big problem for young families is the lack of high chairs or even booster seats. We brought our folding chair, as we often do, after phoning ahead to see what was available.

The bill came to $15.66, reasonable, we though for three adults and two children. We finished in an hour, leaving plenty of time to walk along the C & O canal just two blocks away.