We've stayed at Seven Seas and loved it - but, they have only one bedroom units. The sofa folds out into a hide-a-bed, which works ok for kids. Their pool is small, but, great for relaxing.
If you want two bedrooms, Paradise Villas has received great reviews, but, it'll cost you more... We've never stayed there.
Wherever you decide to stay, you'll love San Pedro. Just understand beforehand, that however nice the resort, it will still be somewhat "funky." For example, all walls at Seven Seas are concrete and 1/2 of the units have no finished ceiling - the rough finished beams are exposed. Cabinetry is locally made, without many of the finish details typical in US homes.
Although San Pedro is more touristy than Caye Caulker or Placencia, your kids can still have a cultural experience. Our teenagers loved it - it gave them an appreciation for a simpler, less hectic life.