I'm thinking of something about 3m in length, as we have two children
(small at the moment, but they'll grow!), so big enough for four is
the requirement.

Intex Seahawk Sport 400. 2.97m long inflatable, which isn't too far from 3m.
Two sets of rowlocks, takes up to 3.5hp outboard (with optional transom). I
wouldn't like to sail it! 4 people will require the outboard, as you won't
have space to swing the oars. It is rated to 300kg and is reasonably bulky
for one person to carry, but possible (I know!)

Goes for about 100 quid new on eBay buy it now (without transom or motor),
or 300 quid in the shops. Ensure it has the "Sport" bit, as there is a non
sport version which isn't as good.

Christian.