I would think if you construct a table at the appropriate height for wheelchair bound youngsters and put an edge around it to form a box, it would work. From what I have read, you should make it so folks can easily reach into it without straining and make it only about 4 feet across at the most. I think if you are dealing with children, it would probably be even narrower than that. I also think the pressure treated wood would be okay if you line the table with plastic, which would also help to preserve the wood as well.

Here are some good links to check out:

http://www.mdausa.org/publications/Quest/q31garden.html

http://www.newmobility.com/articleViewIE.cfm?id=385

http://www.hort.vt.edu/human/pub426020d.html