ferntwist and a 4yo is safer to stand than any of the able bodied 18,25, 35yo who are usually mixed in on public transport?
WHY?
I dont' get it.
Luckily in real life,where I live, this is not how it works at all! the 18/25/35 yo should be standing for people less able to stand, which includes pregnant women, elderly people, people with obvious mobility issues, and yes, little kids!
Other point is.
I have dodgy legs so it's comfier to stand than have a child on lap.
So we get on,3 seats.
Gets crowded, I stand, 2 seats.
Gets more crowded, 1 seat as they share.
Gets more crowded, no seats for the family and a load of healthy 30yo chaps sitting down.
Yes I have done this in the past as I read the threads on here and I don't want to incur the wrath of some of the posters on here, I was suffering with depression and anxiety after the kids and for some years that made me really hyper-aware of us "being in the way", but it does strike me as nuts that a woman with multiple kids > all should stand 
I still stand up for little kids on the train and so do most round here, I will reiterate, and that is good and fine and normal.