I would say ANY game on the 2600 would be cool for kids of all ages, minus a few exceptions like titles from Mystique, Playaround, Wizard, and the other adult-targeted carts.
I agree the Sesame Street related games are excellent for kids (they get to have fun and you get to be all nostalgic with the Atari LOL ).
If you also have a little girl, Strawberry Shortcake Musical Match-up's would be ideal for her.
Good luck!