I built an HA Kits model with my 8 year old grandson. There is a blog here:
We assembled the hull and cabin from the pre-cut ply in one day using superglue - without getting fingers stuck! He really enjoyed that day.
I did the painting and fitting out with motor and radio gear largely without his help as I think that would have been a step too far for him.
He does enjoy sailing it though!
There are a range of different boats available from HA kits.