Its a decision only you can make - the rules are clear; that if the child is over 3 years old, then you should purchase a ticket.
If you are asking how likely it is you will get caught, then you probably know already, from previous trips, that this is fairly unlikely.
Would anything really bad happen if you were caught ? - probably not.
The question I'd ask is...are you comfortable breaking the rules/ telling lies etc. ?......You may be..you may not.........I'm sure you will get people that will argue either way, for and against, the idea.
Without wanting to sound as though I'm preaching (because luckily I've never had this dilema myself), I guess its whether you would see that this practice is dishonest, or not....but be assured that most people in your situation have thought about it..... and quite a few have done it .
I'm not convinced you could argue you shouldn't pay because there will be lots he can't go on....because I could say the same about my 6 year old girl.....and there is still plenty of entertainment for very young children - though, it may not be what you, as an adult, want to do !
I would say its really up to you, and your own moral philosophy.