I have 3 teens and we always stay at Caribe Island. It is as 5 star as you are going to get unless you want to stay at the Victoria House. Beachfront condos at Caribe are top shelf though and you have my word on that. Don't forget to eat at Casa Picasso when you visit. You won't be sorry!
The only 2 bedroom cabana type accommodations would be at Ramon's or Mata Chica. Both are pretty upscale (for Belize!) Other than that you would all be in one cabana with 2 double beds or in 2 separate cabanas, possibly with adjoining porch. Cap't Morgan's has 2 bedroom "cabana looking" villas. Turtle Inn in Placencia also has 2 bedroom beachfront cabanas that are about as 5 star as it gets.
Cayo Espanto on the back side of Ambergris is certainly five-star (at seven-star prices). But villas not thatch cabanas.
The closest upscale cabanas for a family of four on Ambergris would be the two-bedroom villas at Mata Chica or the "owners' villas" at Victoria House.
I agree with Barbara that the two-bedroom seafront villas at Turtle Inn in Placencia are extraordinarily nice (with not two but three baths), and Mr. Copolla's villa when available has its own pool and sauna baths.
The deluxe condo suites at Robert's Grove are also as nice as any five-star hotel I've seen (though this being Belize service may not be what you'd get in a five-star hotel). Also, the suites at Hamanasi are also great (built by the same builder who did Robert's Grove).
And not on the beach, but the two-bedroom villas at Blancaneaux are as stunning as anything I've seen.