Taking into account that it's for kids in a charity, I'd go with Ubuntu. I already set up Ubuntu machines for kids and excluding one or two bumps along the way (which Windows XP isn't imune to) it works just fine.
These are my reasons:
1) Windows XP at this point is too vulnerable to malware. I've seen networks in businesses based on XP that were basically breeding grounds for all sorts of viruses, even with paid AV software up to date, and I assume you'll have only free AV protection in them. Also note that you can't use the latest browser from MS so to be up to date on that you should use another one. If I had to bet, I'd say those computers in the hands of kids will be full of malware in a very short time.
2) Ubuntu is easy to use. I'd go as far as to say that it's more intuitive than Windows. Of course, this won't apply to people that lived their whole lives using Windows but since we're talking about kids here, I don't think that should be much of a problem. I'd stick with Ubuntu classic desktop instead of Unity though, since it's just easier to understand.
3) With a centralized repository of software, kids can install new games and other apps without looking in the wild west of the internet and dealing with installers and potentially harmful stuff.
4) In terms of content and applications, a new installation of Ubuntu is much superior than a new installation of Windows. Of course, this is not a problem if you intend to put some work setting up your Windows with more software before donating, like an office suite, etc.
5) No licensing issues whatsoever, no matter how many people use it, how they use it, how they access it, etc. This may not be a problem in your case.