There are "minor" brokerage accounts that are co-signed by the parents and ultimately end up on the tax forms of the parents[0]. I had one at 16, but unsure if there's a lower age limit.
It would be a much better ROI then converting real money to Robux. All my daughter wants now is Robux (even her allowance) so she could buy crappy 3d items in the crappy 3d game she plays. People are actually selling/buying these items on eBay like they did years ago for Everquest/WoW.
Yeah, I get we see it as crappy but it is essential for them.
I liked more how they did before where you would buy yearly subscription and they would put money to your kid account every week or month. Shame they stopped that.
They still offer monthly subscriptions, 10% more Robux for the same price if you're willing to subscribe. I am glad I didn't, my son still plays a lot, but after a few months of him blowing every dollar he got on Robux, he has stopped bothering. Partly I think he came to the conclusion that games which allowed players to easily buy their way to supremacy were less fun to play (who wants to be cannon fodder?)