Well the whole theory behind admission and not charging for babies is the fact that they have seats to fill, and if your child is so young that he/she would be in your lap the entire time, then you don't need to get them a ticket, but if they're going to be in the seat next to you, then you do. Basically, if they sell out all of their tickets, and you brought your kid, didn't buy a ticket for him/her, and he/she is in the seat next to you, then that's one less seat for a paying customer. It's all about money.