Mark I will assume your kid is at the 50th percentile so around 35kg.
The UNO is a very stable and lightly built one strut kite that is great to teach someone, especially a kid, how to steer and change the angle of attack of a kite and it's relation to bar throw. That's the hardest thing coming from a trainer kite for them to get.
But... let's say you only want your boy out in 12/13 to 15/16 max. Even at 35-40kg that uno (especially when they are still learning) won't, can't (it is difficult) for them to get planning and even if they are planning, to hold and length of tack. It's is also super fast and mistakes are amplified.
But a heaviky built kite at 4-6m will also not like staying in the air in those winds.
The best thing I did was put my lad on a lightly built 2011 6m catalyst. Within a very short space of time he was staying upwind and then my job was 95 percent done!! And thank Fck for that.
Line length... 20-23 is fine but a short lines of around 12-15 will help at the start but will become a hindrance eventually. So ...
I'd just get any lightly built 6m, 3 strut wave kite myself. Wave kites sit a little further back in the window and are super stable in the air. Any of them from any brand will be fine ... although the neo might be a tad heavy.
If ya want to be safe and don't mind throwing some coin in it get the 4m uno and a what i said above then sell the uno down the track (because you invariably will).
Also advice ... your son needs to be at it every single day for at least two to three weeks. Until he can stay upwind .. otherwise you will be back at the drawing board again quickly. Gotta get over that learning hump or just don't start at all.
i can vouch for Stella and shi house. I watched the latter spend literally all summer teaching his boy this past summer oh boy it was worth watching g the frustration on his face. Spent a number of hours at the pond in a chair just laughing my silly head off. But now his son is cranking !!
But when you crack it, it is priceless and right up there with the best feelings I've ever had anyway. Now my boy can hold a 7m with boots in 20 plus and is showing his dad up every session and he lets me know it to... little prick ....