Thanks for all of your replies.

I think I have worked out that I need a M3 12mm Phillips pan head screw. Got some on order and they should arrive on Thursday.

No idea how he managed to bend the screw though - he's quite a small kid and there's barely a mark on the rest of the bike. A couple of months ago I had to replace the noodle of the front brake as it had a near 90º kink in it. How do you manage that?!!