The dictionary at
http://www.wordcentral.com/ isn't too bad. It recognises UK spellings but goes via the US ones to provide definitions. i.e. if you enter 'colour' it tells you that it's the British spelling of 'color' but you then have to click on 'color' for the definition.
Personally (as someone who spent many years in the teaching profession), I'd much rather that children used a printed dictionary anyway. It's always readily available, even when the PC is switched off (as it certainly should be when the youngsters are doing most of their homework). It's also far easier to 'browse' when you only know the approximate spelling of a word..
I now make much of my living from writing and I do much of my research online but, when it comes to looking up words in a dictionary, I always reach for the one on my bookshelf. I'd encourage you to get your children into the same habit.
Chris