Hi Michelle
If you want the full Samoan experience you could look at staying in fales. Taufua Beach Fales at Lalomanu Beach on Upolu would be able to accommodate a family of 5 but would your kids want to be in a fale together, otherwise you'd have to look at getting 4 fales or consider using their Mountain View Fales which are on a hill behind their property, they could have something that would accommodate all of you.
I'm assuming all your kids are over 12? A lot of places on Upolu have a "no children under...." policy.
There's also a lovely resort on the coastal road called Le Uaina that could cater for your family.
Others to look at would be Sinalei Reef Resort and Spa, Coconuts Beach Club and Return to Paradise, all have decent snorkelling out front.
If you want to experience nature, waterfalls etc you'd be best off getting a map and hiring a car or booking a tour as none are within walking distance of any accommodation.
Cheers
Deb