I agree that duPlooy's could work ... including their budget accommodations in the Pink House. At Chaa Creek, the garden suites would work great (two suites attached, two bedrooms, two baths) but they are pricey. Ditto Blancaneaux's villas -- huge two-bedroom, two-bath cabanas.
More at the budget level, Clarissa Falls has a large family cabana. Maya Mountain's cabanas aren't large but they do have a "group cabana" with a series of rooms that might work.
I have stayed at most of the jungle lodges in Belize, in many cases with my wife and two kids -- sometimes a little crowded but not bad -- though I guess the fifth person makes it more difficult.