6 year old.... something naughty like the toilet filled with silly putty that makes farting noises then you poke your finger in it,,, pooping reindeer...general joke shop stuff... that he can torment people with or be 'rude' also suitable for males of any age....
11 year old.... would probably prefer the money or a voucher towards a game
6 year old - can't go far wrong with lego
11 year old - gift cards are good. All the good sports stores do them or maybe GAME if they've got an xbox or playstation