Thanks for all the advise. Wow. I got a lot to learn.
The bike came with 20 x 1-3/8 wheels, as you said, stock on a Junior. Still, you have to see my 5 year old to appreciate him. He is big for his age. Tall and lanky.
Based on my expectations, I think he’ll do just fine on the Junior, but I won’t rule out a Mini frame if I come across one for the right price. If nothing else I have a head start on his next bike!