My kids are similar age. We often go to Federal in Citygate for dim sum, get sushi at Taste supermarket for picnic style eating (they also have sandwiches and wraps), there is a new pizza place that isn't bad next to the Zentro bar (this is not in Citygate, but a short walk away) and of course there are the daily and weekend buffets at Novotel.
In addition, if you walk on the bridge that goes out of Citygate (walk by the mcdonalds to get there) you get to a more local mall. If you go down the escalator in that mall, then turn left, there is a local wet market and they have some food stalls and there are also a few cafe style restaurants.
If you want to be more adventurous, have a day out and take the cable car up to the Big Buddha for a vegetarian lunch. The VIP room is great and young kids are usually free.
Another big day out is taking the bus towards Mui Wo, get off at Pui O and go to Ooh La La. It is right on the beach, and makes for a great beach playdate and lunch.
Enjoy!!