I think it depends on the kid and what you think their marginal tax brackets are expected to look like over their lifetime.
If the kid is going to remain single and have lower compensation and maybe not accumulate much wealth, then switching from Roth to traditional if they start to exceed the 12% bracket probably makes sense.
If the kid is going to marry another professional and they'll both be in high five-figure, low-six figure jobs, then I'd think about sticking with the Roth even in the 22% bracket.
A few other things to consider:
1. What do you think is going to happen with tax rates, either in 2026 or later. That would impact the recommendation.
2. If the kid is going to retire early, or have some low income years for any other reason (going back to graduate school, perhaps), then maybe do traditional now and convert during those low income years.
3. If you're going to leave them an estate of any size, that might cause them high brackets in their 40s/50s. Hard to figure out how to plan for it, but something to maybe think about.
My kids have all been all Roth all the time so far.
My oldest may start making some more money (good degree, good company, good career prospects, but one never knows). But he also plans on never retiring because he wants to earn a lot to give a lot away.
So for him it probably makes sense to stay with Roth at least into the 22% bracket and maybe even 24% if he makes it there before 2026. I think I told him the other day that he could just qualitatively switch to traditional whenever he felt like he was paying "a painful amount" in taxes. Probably not the most laser-like advice, but there it is.