I stayed at Bluestone in March and did the ridge ride along the Preselli Hills from Rosebush. It was, and had been, dry but even based on that I can’t say I’d really recommend it, especially with a younger rider. It’s pretty cut up, boggy in places and hard going at the best of times.
I also rode round Canaston Woods in the evening which had some nice singletrack sections but not much to string together, more suitable for a quick spin before dinner!
My wife rode with our toddler round the Llys-Y-Fran reservoir family trail while I did the Preselli route, but it’s not really much of a challenge and pretty short.
The best option would be, as others have said, to do some road routes, enjoy the coastline and maybe do Brechfa on the way there and on the way back for a trail centre experience (if you’re heading east!).