i always say kid, but not 'kiddie'. that's reserved for old lady in the shop round corner, don't mind it when she says it, but it doesn't sound right on someone of our generation.
pmsl at 'smalls' - isn't that underwear?!
'children' or 'child' is a bit formal. as in, arranging with mates to go out and say, "managed to offload your 'child' yet?". you cannot be jokey or chatty with the word 'child'. neither with 'adult'. would rather say 'grown up'.