I had a sports card auction party for my son's 9th birthday. Buy a few packs of sports trading cards. Give each child the same amount of either chips or play money for bidding. Put a post-it one the back of each trading card with a dollar amount written on each. These dollar amounts are what each trading card is worth. Then start the bidding. When all the cards have been "sold" the boys can turn them over & add up the dollar amounts on the post-it notes. The boys can do some trading and keep the cards as favors.