If your child is 3 they NEED a ticket.
If you sneak them in it is the same as shoplifting. It is not getting away with something that is no big deal. It is stealing, how can anyone justify it any other way. Getting away with it doesn't make it okay, if you shoplift from any store and get away with it you have still stolen something. Just because it is Disney and it is expensive doesn't give you the right to steal it. Okay fine I guess I will just steal the watch I like after all I spent enough money there already why should I have to pay for this too. It isn't like I am not going to buy any other souviners, heck I probably will even pay for most of them, so it doesn't matter. Next time we come I will pay for all the stuff I bring home.
It isn't a moral question, it isn't a choice for you to make, it has already been made, 3 and up need a ticket, end of story.
You can't afford to buy a ticket for the 3 year old, then I guess you better save up a bit longer and delay the trip.