I just got an ipod shuffle 1GB from bigpockets for £25 and it really is bob on.
looks quite good, good build quality, good sound, and dead simple to use. No screen or menus but half the fun is not knowing whats next.
Its better than any i have had before and really is tiny and light, the clip built in is probably the best idea yet, even better than Britain's Got Talent.
i think the best mp3 player for a 10-year old would be a sansa fuze (see http://www.sandisk.com/Products/Catalog(1256)-SanDisk_Sansa_Fuze.aspx for more details). it's got everything you would want in an mp3 player, and it plays some videos too (i'm sure your 10-year old would want to put some funny clips in there too). it has got a memory card slot to upgrade the memory of the player, which is always a good thing, and has FM radio. when you get this for your son, i think you would want one for yourself too ^_^
I would advice to buy a MSI player. MSI P100 has 4Gb memory, long battery life and doesnt have an energy eater: display! So- no sratches and extra battery energy loss. It is a perfect player!