Honestly, you shouldn't even be considering XP for any machine you will connect to the Internet. XP is no longer supported by Microsoft, and there are critical in-the-wild exploits for it that you cannot get patches for. XP is also a bad choice from a security standpoint for other reasons - its last notable update in terms of security features was almost precisely 11 years ago, so it's missing all the modern security features - but it was at least a justifiable choice while Microsoft was still patching it.
Just how much RAM do these machines have? While Win10 can technically run on under a gig, I wouldn't recommend it (certainly not for gaming). For 1-2 GB, Win10 (or Win8.1) does very well in general, though it's still underpowered for a gaming box. For more than 2GB, you'd be a fool to put XP on there anyway; XP's memory manager was designed for a time when 256MB was a lot, and it does not make efficient use of large amounts of RAM (remember, unused RAM is wasted RAM. Newer Windows versions use free RAM to do things like pre-cache files they expect to use soon).
Of course, using anything with less than 2GB of RAM to run modern games is a terrible idea anyhow. LoL might run in 1GB if you turn the settings down enough, but it won't run well on so little RAM, regardless of the OS.