Would help if we knew what country you were in since presumably local would be better. I'd think any of the manufacturers who currently make jumpsuits in child sizes would be willing to customise them, but beware that this would probably be expensive depending on the level of customisation. Custom colours are a standard offering, but if you want to change the design of the suit itself that could get prohibitive.
No harm in asking though! The only manufacturer who I know produces jumpsuits for kids is Bev Suits. But I expect there are others.
My wife has done them. And all of Mike's comments are correct. If these are for specific and serious flyers - then these are more difficult that suits for grown ups to get booties and torsos to fit just right. they have to be dialed in very well - local builder is an excellent choice
unless the buyer is just looking for something like what the tunnel has for the general walk-in crowd - then they might be easier to make those - they are just reinforced coveralls with special closures - then it would just be colors and logos - likely much easier.
Oh, and of course whatever your 'costume' design, a key consideration will be safety and flyability in the tunnel. Since you're talking about a superhero theme, it's worth pointing out that capes will not be welcome!
Unless you already have some involvement with the wind tunnel operation, you should definitely consult them before you get into this too deeply.