Bit late but…
Kirroughtree is an absolute MUST. The Black is probably the best trailcentre I’ve done, although I’ve not made it much further North. The blue is really, really good fun – it’s easy enough for beginners but it shares a lot of sections with the red, so there’s plenty of fun to be had for more experienced riders.
In the same area, Glentrool blue is good fun, but not as much so as KT, and Mabie red is definitely worth a shot, I’ve not done any of the more extreme stuff there. Apparently the blue is all fire road. You’ve also got Dalbeattie, Ae and I think Drumlanrig pretty close, so you could easily spend a week there and not get bored. You should also visit the Mull of Galloway, and there are lots of (not too big) hills to explore.
Others have pretty much covered Glentress and Innerleithen, but I’d reiterate not to discount the GT blue – it’s like the KT one in that there’s lots of fun to be had.
Personally I wouldn’t bother with Dalby, I’m not a fan of the place. Might be worth stopping in the Peaks on the way up though, if you do a loop from Ladybower there’s a visitors centre and stuff to entertain the family while you go for a ride.
Other places to consider would be Wharncliffe, Grizedale and Whinlatter off the top of my head, but I’ve yet to ride any of those.