Kids are Kids are Kids Technotiger and i think if you actually just gave them the PC they would learn how to use it, the user guidelines which are solely down to the parents i would say must be adhered to it would be easy to sit them down and forget about them. As for Pc itself set them up a limited account on their mums that way you can limit the use easier..
Depends if you want to go for a desktop pc or a laptop. If a laptop then spmething like click here cheap, but will do the job. If you after a desktop pc then try click here No monitor, but you can pick one up in Tesco's for £100 Hope this helps.
personally, i would get a half decent 2nd hand one,ive seen a few "whats it worth posts",the spec has been good,and more than adequate for the formative years
my 7 yr old wanted a pc so she and her sister didnt have to share,so i picked up a half decent second hand one that had dvd rom,on relatively quick processor and 512mb ram,incl os,monitor,mouse keyboard,speakers and software for £120 and networked it
she happily clicks away,watches dvds and surfs the net,loves music and occasional word processing her only request was not slow like grandads!