Boy does this bring back memories...

When I was her age, they took me out of one afo, and left the other foot braced (why they'd do that to a diplegic, I don't know). We were super-duper poor, so mom couldn't always do the two pairs of shoes method. Her solution was big thick socks on the unbraced foot, and hoping for the best. On occasion, she bent the law. I remember once she just switched them in the store, and told me never to tell... that it was ok because we needed to do it. Back then, they weren't as militant about checking matching sizes as they are now. And another time she just put the shoes on me (in the two different sizes) and took the empty box to the cashier, asking her if I could wear them out. That worked too. Not that I'm condoning "breaking" the shoe laws, or anything

I'm bilaterally braced now, and have found that the best athletic shoes are that stretchy fabric-ish netting material rather than leather. Saucony makes shoes like this. Not sure if they make kids' sizes, but might be worth a try. There's more "give" and you can tie them snugger than leather sneakers. Even though my feet are the same size now, one brace is a bit bulkier than the other.

HTH.