Whatever you decide I would end while still in the single digit levels. I've never played it before but dragon's demand might be a worth a look.
As far as difficulty goes just add to the number of enemies and give max HP.
Kingmaker is probably a pretty good one, although the kingdom building stuff might be a little bit boring/math intensive for the younger ones to participate. It has a whole lot of 1 a day encounters, which are pretty easy anyway since everyone has full resources, but using the six player changes or simply just doubling up most encounters is workable.
One thing you might want to do is make sure there is more 'room' in dungeons etc, so that everyone can participate. Some of the encounter areas are a tight squeeze getting 4 PCs into the action, which is a fine tactical challenge, but you don't want 3 out of 7 of your players unable to contribute.
Encourage them all to be Ratfolk, that swarming ability is GREAT in melee, but still works even for casters... :D