i have done it and my parents did it with me for years, i know plenty of people that do.
when we went to legoland we were already paying for four adults and two children, dd was free as she was a baby but ds4 was just 3 and so we 'should' have paid, i simply carried dd in a sling and put ds4 in the pushchair, took his shoes off so he looked younger and gave him his fave blanket knowing he would then suck his thumb.
we still paid a fortune to get in, these places often charge extortionate prices!
re the talking thing i remember a friend and i were both pretending our boys were under 3? and again had them in pushchairs and gave them snacks/drinks so they didnt talk as we went through the barrier.
is it the moral thing to do, probably not but there isnt much moral about the prices they charge for some of these places.
and lots of places do family tickets that are for a parent and 2 or 3 children, we have 5 and we cant get a family ticket for us, so end up paying individually for all of us, we have tried to get a family ticket and then pay for the extra children singly and been told we are not allowed to do that!
and yes i have tried to buy tickets ie on a train journey and been TOLD by the conductor to say they are under 5, so it works both ways i am sure hte parks etc are WELL aware this goes on but they woudl rather have your custom than kick up a big fuss.