In reply to MattH:
Easyjet are relatively cheap to add a bike as luggage, and they aren't very fussy as to how your weight is distributed amongst your separate items. Might be able to get two kids bikes in one box? Can always gaffa tape two mangled boxes together if you need to extend it slightly, but that adds a bit of weight.
Cardboard boxes from the local bike shop are, in my opinion, definitely the best way to fly. Every bike shop is happy to get rid of them, they do the job, and if you need to dump it you can. There tend to be two types, slightly smaller ones bikes come in from the factory, and slightly bigger ones that are common with ex demo bikes, and therefore rarer.
If you are moving a whole family of bikes, a friend once told me that getting a standard pallet shipped is very cheap. Could work if you can plan in advance...