I think maybe enough tickets for each kid (except the babies) to play about two or three games would be nice. As for food, you are providing cake and they can always leave and eat dinner if they want at 5. Maybe you can also bring some drinks as well, since they can use the tickets to play a game or get a hamburger instead of drink.
Maybe say, $5-$10 worth of tickets for each family... depending on family size? Just think about how much money you are saving by not getting lots of decorations, or hiring entertainment, and party favors and food if you would be having it at your house, and use that as the ticket amount. I guess, it really just depends on how many families you are inviting and how many kids they all have and what you can afford.