ideal families

I have heard lots growing up about 2.4 children. I'm assuming that means
that if you are half way through your pregnancy with no 3 you have an
average family.
One thing I have found though having 3 is that if you go for a family ticket they ask for the ages of your children, not something I've really been asked before. When I ask why thinking they're checking no-one is over 16 they reply that a family ticket is 2 adults and 2 children. I think this is a bit of a swizz really. I know it's open to abuse if you have unlimited children on a ticket, but surely there would be some way of sorting it like bringing passports with photos on or something. At the mo our no 3 is quite tiny so we don't have to pay, but once we do it will get quite expensive. I have lots of friends with 4 or more children and can understand why they struggle to do lots of places in the holidays as it's just so expensive.