Firstly, all fast food chains like McDonals, BurgerKing & Hardees arrange birthday parties for kids. Pizza Hut do not arrange in their premises.
They have nice economical packages and kids have lot of fun and gifts.
They also make movie and give U a DVD of it.
Better make a trip to any one of them for more realistic scenario.
If you want in a vegetarian restaurant, then try ASIANA near Airport Signal. They have good hall and music & video arrangements too.
There in Asiana, U can chose a vegetarian menu as per your desires.