Would the better advice in this kind of situation not be "slow down so that you can avoid hitting small children" rather than "wear a helmet so it doesn't matter if you do"?
No. The OP didn't say that it didn't matter if children were hit. He also didn't say that helmet-wearing would guarantee no injury to the cyclist. He simply said that there are times when things happen that you have no control over.
There are situations where small children, big children [***], small adults[***], big adults[***], dogs[***], cats[***], squirrels[***], wheel-jamming bin bags blown by the wind[***], debris falling off skip wagons[***] etc. suddenly appear immediately in front of you through no fault of your own and you cannot avoid hitting them, other than by never going at more than walking pace and
even then collisions can be inevitable.
[***] I have experienced all of those. A rabbit, squirrel, or yes - even a small child can be out in the road before you know it. It can't always be anticipated.
A friend of mine was once driving slowly through a village in Wales when a small child suddenly ran out between 2 parked vans in front of him and he hit her. She got to her feet and ran back into the house from which she had just emerged, pursued by my traumatised friend. It was a hot summer day and her parents had left their front door open to let a cooling breeze into their house, one of those whose front door opens straight onto the pavement. They had turned their backs for a second, and that was it - she was gone! Fortunately she sustained no more than cuts and bruises. The point is my friend is a careful driver, was driving well under the speed limit, but still could not stop in time. It was not his fault.
Yes, ride carefully. Yes, anticipate that children seen running on pavements may suddenly run out, but there are times when you might be caught out by one that you could not see, such as in the example I gave above.
I think it is insulting to suggest that somebody discussing wearing a helmet is more worried about their own head than about hurting children!