Not sure what car you have but transporting an air floor SIB on the roof fully inflated (we have a 110 Land Rover) is a piece of cake and just about the most practical and fastest way to go boating, from parking by the beach you can be offloaded and on the water in 10 minutes. We have a 3.4m and leave the launching wheels on - easy for two adults (myself and my very slight wife) to lift up (slide it up the side of the vehicle) and secure with 4 ratchet straps - solid as a rock. Pull up on the beach, undo straps, flip the boat off and ready to go.
A 3.8m Honwave will lift up easily - I would go for a 15hp *2 stroke* Yam or Tohatsu as the perfect partner - a 4 stroke 20hp weighs a ton.
With two adults and presuming the kids are small you will be OK with a ringo but not much else is really practical to tow behind a SIB with that load.
Airfloors are great if properly inflated and obviously (?)
the kids will be wearing sensible flat 'boaty' shoes - do run a search on the forum though - loads of previous topics on 'which boat' etc.
I briefly had a 3.8m alu floor Yam - totally different beast to the air floors and weighed a ton in comparison - far too bulky and heavy to manhandle.