We've just finished reading Pat Brisson's Hot Fudge Hero and Bertie's Picture Day and First Son and I loved them! In Hot Fudge Hero, Bertie makes friends with his grumpy neighbor (and his scary pets) to win a bet. In Bertie's Picture Day, Bertie returns for an eventful weekend before picture day at school. (It's certainly one to remember.) I just love the relationship between Bertie and his younger sister. They have their disagreements, but Bertie is a good and kind older brother, just the sort of role model I like to present to First Son. These books are well within First Son's reading level, too.
For those who worry about such things, one of the stories in Hot Fudge Hero has "fast magic" and "slow magic."
I think these are out of print, but our library had them. They're worth a little seeking out (as is Little Sister, Big Sister, which I have read but not yet to the kids).