You could always make up something on the computer; a prayer card or something. Some sort of memento to commemorate the occasion.
In the church I belong to now, baptism occurs at about the same age as a first communion, and my daughter recieved a necklace from her aunt for that. I suppose it would be a bit more difficult to find something for a boy.
In my family , no one would be offended if they recieved a card with out any money, because they would know if a family member was strapped for cash. I think five dollars would be sufficient, after all the child is only in second grade, right? If that's more than you can go, a dollar or two would surely be appreciated by my children. I surely would not worry about insulting anyone with a gift...it's a gift, not an obligation.