You might be amazed at how little power you can get away with in an actual tractor. 20HP is close to the upper limit of practicality unless you are running a large commercial operation.
I'm a farmer, I know what working vehicles are subjected to over time, and I know that when plywood gets wet, it swells, warps differentially, splits at its layer boundaries, and starts to twist. Tractors are for driving over land that is often at least damp. This is not a recipe for durability.
Literally children die from riding on such things.
Keep your children safe a well and alive, dont let them on anything like this or near one being driven by someone else.