The Untimely Adventures of Mimi - Book 2: George and Martha Washington by Edward Bloor and Amanda Breed

Mimi, Benjamin, and Percy are at it again! When a nasty British nobleman claims ownership of a portrait of George Washington —a claim that could cost Mom her job and the College of New England one million dollars— the time-traveling trio head off to Mount Vernon in the year 1797 with a bold plan to change history.

Really, what could go wrong?