Hiking from a base camp in RMNP – will gear be stolen? Should we stash it somehow?

I think you may be mixing modes here. Leave the expensive ul gear at home if you are doing that kind of trip and just use a cheap heavy tent that will be more comfortable anyway.

Sorry it was confusing. We are building my son up to doing longer trips so we want to use our normal gear but shorten the distances so he finds this a fun and not arduous activity.

It has never happened to me, but I've had friends have stuff stolen out of tents in the back country. No way to say how likely it is or not, but if you're worried you could bring a green or brown dry bag and put all your stuff in it and hide it somewhere nearby.

"Wondering if we should re-think our plan and just hike through each day to a new site."

If you can do a loop that would solve your problem, and it would also provide your son with the very valuable experience of finding a campsite, making/breaking camp and preparing dinner and breakfast. Just keep the distances between camps appropriate to your son's age and fitness level, also taking into account altitude and water availability.

That's the beauty of UL – your packs will be lighter than a lot of dayhikers' daypacks!