I’d agree that you may be overthinking it.
That said, is it the endurance or the technical stuff that causes concern?
If endurance, then Swinley red is both pretty santisied and fairly short, so well worth a trip.
If technical, the Gorlech red at Brechfa is not too challenging, and not too long, but the fireroad climb to kick things off may be off-putting for a little’un.
FoD red as above is excellent too.
But, all things considered I’d just let them crack on with any trail as above.
I’ve just come back from Italy where my 12 y.o. daughter and I hired bikes for a jaunt from the campsite. It was flagged as a ‘medium’ trail. After a gravel track to kick things off, we got to a rock garden ascent that wouldn’t be out of place on a double black diamond in Canada. It was followed by some rocky descents that were rougher than Coed-y-Brenin blacks.
My daughter is no expert, and pushed up the ascent (as did I!) but nailed the downhills without so much as a peep from me.
What’s more she did this on a city bike with 700c tyres, a step through frame and about 50mm of front suspension! I’d love to claim this was as a result of all my skills training over the years, but she just rode her bike.
Just let them crack on!