Books - find something appropriate but clever for that age, write a personal message inside the cover, done.
For people saying cash the parents would be pretty unlikely to let a 6 or even 9 year old just keep and use cash if it were given (assuming they were basically competent parents). The kids would probably just buy a bunch of candy, make themselves sick, and not really associate it with the giver at that age if allowed to just take it to a store on their own.