When my kids were pre-teen and even when teen, their bikes were left out in the elements more often than not. They worked fine even when cogs and chain were covered with surface rust. Of course they were growing kids, so the bikes were thrown away and they got new every three or four years.

Maybe you can buy cheap bikes too and do the same. Throw them away to be re-cycled when the rust gets to be more than just surface rust. But if you spray a light lube on the rusty cogs and chain periodically, I'd bet they will last quite a few years.