I'm a new pilot (passed my checkride last Saturday). I have a wife and
three small children (all 6yrs and under).

Weight and balance would be OK to fit three kids in back of a 172, and it
looks like plenty of space (at least while they're young). Is it possible
to get a STC to install a third seatbelt?

If not, is there any other fixed-gear single in a similar price range that
could accomodate 5?

Thanks,

Adam