Yet when I look at my three year old toddler and deem that he will not be able to use the facilities anymore than he was able to a week ago when he was two, then I am a big old dishonest thief?
You need to look at it the other way round. At 2 years 364 days the reality is that your child is able to use the facilities well and get a lot of enjoyment out of them, which is no doubt why you take him there in the first place; and it's a total bonus that you don't have to pay. Yes, a couple of days later his enjoyment is much the same and you have to pay - but there has to be a cut-off point somewhere, doesn't there? The reality is that if they don't make money out of the facility, it won't exist, and children won't be able to enjoy it free or otherwise.
I suppose the question is whether you want to live with the stress of knowing that, if someone finds out, you will get chucked out publicly and will have lost the benefit of the adult tickets you paid for; or, if they decide that they want to make an example of someone, they might decide to get you arrested.
Nickifury, it isn't comparable to what you describe as "sanctioned thievery" of the organisations in question, for the very simple reason that no-one is making you use their facilities. If you don't want to pay, don't go. There's plenty of entertainment out there for children that doesn't cost anything like as much.