I'm a huge fan of Larry Gonick's "Cartoon History" series. I read the earlier volumes a good dozen times as a kid, and even bought the final one when it came out recently and loved it just as much. I definitely credit it with being a huge influence on my love of history. The main word of caution I would offer is that it does have two major shortfalls - While researched and with a long bibliography, in some cases the scholarship is just plain out of date, especially with early volumes done in the 1980s. Secondly, more than a few pop history "theories" find their way in. But all in all the faults are ones I think can be overlooked, especially given the age range.

The Horrible Histories would be appropriate if you like the style.