Hm. I think it's kind of a gray area but could likely be ruled a violation. I can guess who you're referring to, not sure it needs to be a secret since they are so famous now esp in the gymnastics world, but maybe the mods prefer not the advertise. NCAA generally prohibits "pay for play". Designing and selling leotards probably can be done without crossing that boundary (although you would have to avoid some things). More problematic would be paid advertising on the meet videos or pay per click stuff. I'm not sure how that works (from the YouTube end), but I imagine it could be considered "pay for play." Also at this point this family has one of the most successful YouTube "franchises" around and seem to have made a lot of money...so they, unlike most people who will never face that payday, probably decided to weigh the odds of their children receiving an NCAA scholarship against the payday now that will likely finance their educations. I would think this would be more problematic for the less successful but still semi successful YouTube gymnasts (successful enough to maybe make a little money).
Still, a little uncharted territory but I think we'll find out soon. Current NCAA athletes wouldn't have had a lot of time to establish YouTube channels prior or during the start of the monetization of private YouTube channels. The closest I can think of is Nica Hults, whose mom had some very high view videos of her when she was young (around 7 or so) that have been viewed a lot on YouTube. I don't know if they elected to make money off of it, but probably could have. Lizzie Leduc is another example. Neither are comparable, but probably the closest I can think of that had high view videos a long time back. Obviously they weren't ruled ineligible, so...? Either they didn't take money or it wasn't enough to come to anyone's attention or there's no firm precedent yet and they got lucky. It will become a much bigger question in the within the next five years so others may not get lucky.