Going to the 20 is like getting off a scooter onto a Ducati 900 grand prix bike in the wet with a sticky accelerator.

If you flip, and you will, just stay with the boat and keep your son safe.

In 12/15 knots of off shore wind that pop up on a quite day you will not get home.

I would suggest you get a ride on one and flip it before you buy it. If you want your 5 year old going over the top of that arc, go for it.
I am a firm believe in an attrition rate to life.

Check out the comments of the AC mono guys about multis.

Think about how much of this choice is about your ego and not your boys enjoyment.

I don't want to be a total kill-joy so I will say that if you have some GOOD skills you might get away up to a 16 with some judicious decision making before leaving shore.

Unfortunately the H16 is over powered and hard to right so a more cultured 16 would be a good choice.