After making my grandson's Airport Loft Bed, he said he needed a different chair for the desk inside. But, he wanted wheels and he wanted it to go up and down and turn. All of the kid's rolling pneumatic chairs online were way too expensive and they didn't go low enough to fit under the desk; so I decided I would have to build it - just had to figure out how. Fortunately about that same time, I got an ad for Harbor Freight with a Pneumatic Stool on sale for $19.00 with the coupon.
The stool could have been used as is, but I wanted to maintain the wood look and have a back to the chair. I built the seat from wood scraps - removed the cushioned seat from the stool and just fastened the new wood seat, which is 13" square. It adjusts from 15" to 20" and holds up to 300 pounds; so he shouldn't outgrow it. As a bonus, he can store some of his airplanes in the rack at the bottom.