You lie about the little things and it becomes easier to lie about the big things.
Bullshit
Is it, though? He's only one day over the threshold, so that's nothing. What if something comes up and they have to postpone it for a week, or two, or a month....? What if he has a whale of a time, wants to go back for his 4th birthday and you ask yourself why you should be expected to pay all of a sudden when nobody quibbled it last year? If you disregard the clear rules that have been set, how do you set new rules that you find an acceptable bending of the given ones, but no further than that? Or do you just push and push with what you can get away with until you're stopped/shamed/arrested?
It's not a simple case of little thing vs big thing. It can build up gradually. Once you've broken the small rules/laws, it's easier to break the very slightly bigger laws, and then the medium ones.... Their age limits aren't just something they vaguely advise, they're their own actual contract of business. It's the same principle as the prices they would set for their cafe - if it's self-service and you put most of your food on the tray for the cashier to see and charge for, but you take an extra sandwich and shove that in your handbag - is that OK? What if your child desperately wants something in the gift shop, but you don't think it's worth the £10 price tag (or you can't afford it)? You would have paid £6, but because they are so 'inflexible' and that isn't possible, do you just slip it into your pocket - or tell your child to do so?
Once you reject the official rules/laws and get away with it, it's the easiest thing in the world to push it further, little by little, and tell yourself that you're only pushing it one little step on from the last thing you got away with rather than comparing it with the original clear benchmark and looking how far you've come/fallen.
Also, regardless of the business you're stealing from, it's a terrible lesson to teach your children. When you're very little but now able to talk and rationalise simple facts, how old you are becomes a very important part of your identity. Telling a child to lie about it, or gainsaying them loudly with a lie when they proudly state the truth, is not a lesson I'd want to teach my child. Apart from bashing away at their fragile little confidence and self-esteem as they're only just developing it, once they know that it's acceptable to lie, you know exactly whom they're going to be lying to next....