Would it be appropriate to charge guests to a childs party (which is optional if they want to play)?

The trick here is that you're putting the parents in an uncomfortable situation. The little kids, faced with all the fun things to do in the inflatable party area are going to want to play! So why invite them there if they can't? No, it's not appropriate. You could invite them to your home for cake and so forth just as easily. This either forces the parents to pay or tortures the poor little kids!