Hi Andy, you would probably be better going for the bigger sized 11'5 version.
The 10'5 would float you and be ok, but the 11'5 will be even more stable for teaching friends/nephews on and certainly offer you more float for lightwind freestyle.
We had the 11'5 here as well for a short time and when planing there was very little difference in performance between the two (probably due to the stepped hull design).
The other option would be to go for a Fanatic Viper style of board (suitable for lakes and teaching etc and a lot more compact), but if you like short board/ wave sailing, you will prefer the feel of the Kona when planing.
The 11'5 would probably be best, but bear in mind the length - they arent short! If you have a van, make sure it will fit.....