For mini pizzas you can also buy English muffins and just have the kids put on the toppings they like - put them on a pan, throw them in the oven and 20 min later - voila - mini pizzas.
Instead of having a b-day cake, you might make some cupcakes and give each child two to decorate - although this could get messy if the kids are in their pjs, but you could ask the parents to send each of them with an apron or smock to put over their pjs. If you have a digital cam, take a photo of each of the kids holding their cupcakes and just print it out on paper for them to take home. Cheap and fun.
You could do a "cooking class" for the girls. Have them break into two groups and learn how to make something fairly easy that kids would like, then they get to eat it. One group makes home made garlic crescents and another makes sphaghetti.
The girls could play pictionary with easy words - that's always a blast. Just buy a write on/wipe off board and put say 10 words into a jar - break up into two teams.
Some fun and cheap games include: balloon stomp, musical chairs, pin the birthday hat on the picture of your daughter/son, drop the clothes pin into the small-mouthed jar (they kneel on a chair and looking down have to drop it into the jar on the floor.) Bean bags into buckets. For your son, maybe something along the line of your favorite superhero - have them draw what superhero they would be. You can go buy some super cheap different colored fabric and cut it into capes, them have them draw with marker on the cape.
Hope that helps!