Well it does take a bit of practice and you only have to get it wrong once! It makes a mess of your trousers and that is no good if it is cold.
I bought one and have the confidence to use it at home on trial but have yet to summon the courage to use it on one of my rambles. Also it is only very rarely that we go for a walk without some kind of loo stop en route.
With regard to a child using one instead of going to a campsite loo, I really think a bucket would be the better option.
I've never managed to get mine wrong, the shewee really is shaped so that it's quite easy to use, it really is just having the confidence to use it whilst fully clothed!
I have only used the sheewee so can't comment about other types as someone asked earlier.
Romany, I agree for kids and for most people with enough mobility a BWL is easier and less hassle to use in a tent environment.