While I think of this as more of a problem of biomechanics first, judging/aesthetics second, it was my all-time favorite brevet judge who first brought this toe-out position issue to my attention when I was a much younger coach. She was scolding a new elite at a regional training camp for keeping this bad habit all the way through JO and never attempting to correct it until she got to elite. That always stuck with me!
I consider it to be a nasty habit that will show up elsewhere as well (especially on bars). It's also the hallmark of a weak runner/tumbler (while slight toe in is the hallmark of speed). There is something going on that is creating that shape that can be different for different kids. Sometimes it's weak ankles/arches, but other times it's a hip thing that's causing the chain reaction down to the foot. I have had kids who do this work on the strength issue (whatever it may be) on the side, while I nag for the correction as a coach. I don't try to diagnose the root problem myself, I have a professional do that. From the coaching point of view, it's best to correct it during conditioning where there is no fear/risk involved. An example is having them do their candlestick jumps with a hair scrunchie (or something small like that) held between their big toes to reinforce where the toes belong. I don't think it's effective though unless done side-by-side with the targeted strength exercises.