WELCOME to the forum!
A flip-up, back jump seat from a small extended cab pickup comes to mind. They are small, they flip up and already have a bracket that could be modified. A junkyard run should score one.
Just don't mention here that you want to put a kid in it. It is an odd group of people here that I thought (with all my time on the old ford forum) would be get-it-done farmer types. The Kubota market is completely different, there are a lot of urban leaf movers, arm-chair lawyers and nannies here as well as a few farmers.
The ladder group will say go for it, teach away. The former group will say you should live inside the condom until you die. And then there are the passive aggressives - "Do what you want it is your tractor - but don't come back here and complain after you kill everyone".
I was taught on a tractor by my grandfather before I even remember doing it. So I say two thumbs up
. There needs to be more "tractoring" taught to the young these days, not less.