I've more or less settled on NFS: Hot Pursuit for Nephew.  I think it ticks the right boxes and I can always lend him Forza 3 when I've finished it.  The next question is whether to get my fickle niece a game also.  Apparently she mght like a DS game, she is only 6 or 7, and apparently likes playing on her DS - but I really have no idea what sort of games 6/7 year old girls are even remotely likely to like...
 
Grrrr!
 
At least I had an idea with the nephew!  Has anyone got any suggestions or am I just best sticking a £20 not in her card?