Which plane for 5 small pax?

I have the opposite problem from the usual "four seats doesn't
mean four pax" problem -- I'm looking for an aircraft with typical
four-place load (and price) to carry five small passengers and
minimal baggage. What aircraft are out there on the used market
(including hombuilts!) which can safely, legally, and economically
carry two small adults (150 lbs each) three children (projected
total weight in five years at ages 9,7,5: 170 lbs), 100 lbs
baggage, and reasonable fuel, say 3 hrs? (570 lbs + fuel)

On the certificated side, it looks like a Skyhawk or its ilk would
(barely) carry the load, but I understand there's no legal way
to squeeze three kids in back of the four-place Cessnas and Pipers.
(Why not? Surely it's not that hard to add seatbelts!)

What else is out there in the world that I haven't heard of,
without moving up to a six-place plane?