I have a niece and nephew who are twins.  They turn 7 next month, and I would like to start purchasing a chapter book series for each of them.  Ideally, I will buy them 2 or 3 books in the series each year.  So, it should have a half-dozen or more titles in the series, and it should be something they won't outgrow too soon.

I need one series for each child, one more boyish and one more girlish.