Hope this helps:
For the girl you could get mini nail varnishes, lip glosses, little scented smellies like mini bath bombs/heartshaped soaps, mini perfumes, a coin purse-maybe stick some choc money in for some fun- (nobody is ever too old for choc money), any books, obviously sweets & chocolates, earrings/jewellery (Claire's accessories have constant deals on), iTunes gift card, mobile
Phone case.
For the boy (sorry i'm abit useless with boys seeing as how mine is only 8 months old! but here goes:
Maybe a keyring of something he likes eg: marvel or a favourite team/sport: look online Amazon have great selection), novelty socks, gadgets like light up pen/That kind of thing, a wallet, iTunes giftcard little male grooming kit(they're never too young to start), headphones, mobile phone case, any books/card games/puzzles, sweets& chocs.