Have not been to Cwmcarn, we did a weekend in Afan last month. I dont think there is much there that a relative beginer needs to be worried about, I cant actually think of any really sketchy sections, just a few little drops they might want to walk over but could ride if they knew they could (if you know what I mean). Was a fella out with his 9yr old just taking it nice and easy and having a whale of a time.
I wouldnt do Skyline again, lots of zzzzzzz forest roads between singletrack though that might be just the ticket for your pals. W2 (Whites and the Wall combined) is a good trail and you have the break at Glyncorrwyg if you start at the other end.
Fitness will be the main barrier to big fun day out I reckon. Both are a fairly long way and the climbs out of the valley are proper ones.