The Galaxie Diner is a really cute and cozy retro diner. With only a handful of tables, (and thanks to its yummy menu and great prices), you'll most likely find a line up outside the diner on Saturday and Sunday mornings. Its location is great, just off of 17th Avenue (lots of great window shopping). I think it's a great place to take kids for lunch; the atmosphere is fun and interesting, and it's fun to watch the cooks flip the hashbrowns on the grill.
The only downsides I can think of are that it's cash only (not sure if it still is), and I've had blah service on one occasion.
Tips: They only seat you if everyone in your party is with you. Try the chocolate milkshake because it's awesome!