Hi 20hp is plenty for that I would think about a 15hp 2 stroke as come in at about 36kg which is plenty to cart around!
With the zodiac fast roller you very much sit on it not in it, so kids will be much more exposed.
Something like the Honda air floor gives you much more depth in side giving you a more secure feal same as my Bombard which is a great sib
If you do not need it to pack away then I would defiantly look at a small rib!
And you would be hard to go wrong with a 4m sea rider
and if you are looking at keeping a sib on a trailer they rather defeat the idea of something being able to pack up.