First you asked about a one hour kid's show. WAAAAYY TOOOOO LOOOONG!!! With their attention span go for a good 30 minute show, any longer and you have some bored kids that will ruin a show for you.
If you're starting out it's probably good advice as a 1 hour show might be biting off more than you can chew.
If you're an experienced entertainer and the kids are getting bored after half an hour, I'd have a serious look at your show.
A 1 hour show is pretty standard in the UK. I do a 50 minute show as I found that was a good length for what I do. Sometimes a booker requests an hour and I have no problems extending it to that length.
I don't work an hour long show, more towards the 30-40 minute mark, but I have watched some magic shows at an hours length and while some failed, some did indeed work real well. It really is about how you present yourself and your magic.
Kids can be enteratined for an hour, just make sure you have an hours of entertainment.