I think you should seriously look at the Fluid Synergy. It is a double plastic kayak, that can easily be paddled by one person or two and have the capacity to accomodate a 3rd smaller child in the centre seat. Two of them easily fit on a customised roof rack. I have paddled mine in the surf and extensively on rivers, alone and with my son and they handle great. They are rated up to a 2 or 3 for rapids as well. In the sea they are not as fast as the fibre ski's but thet are super stable and with the tri-hull they handle the surf launches and beaching rather well.
Check out their website and the trip they did ..........

Badballie - Sealine Member

Amanzimtoti