Indoor material

[rc13]

I think soil would be best. As long as you have sides built up enough to contain spills. A while back I built a very small play area in my basement out of wood. Wife would not let me use dirt so I used pinto beans... Yes pinto beans. They were inexpensive and worked well for my wheel loaders. But not so well for a track machine, so I kept those out... I basically pushed piles and loaded trucks. Nice to have in the winter for testing. It was a small area and the side boards were only 4" tall so it continued to spill out. So I finally took it down.
If I had the room for a 10 x 20 like you, I would definitely use soil.