Grogley woods in bodmin has several routes but think they are all unmarked ? I went there with mates and just followed them
Cann woods in plymouth, its a forestry place with some well laid out runs but you need to know where you are going, i would be happy to meet you here and show you some routes
Haldon just off the A38 near exeter, several coloured trails that are reasonably safe and well laid out and signposted, trails are quite long so a couple runs will last a couple hours again i would be happy to meet you here as the family quite like it
Woodys bike park near fowey, its very jumpy but if you avoid the blacks then the reds and blues are all tables so safe if you go short and theres uplift so a bit more expensive but you can spend a few hours without dying
Gawton gravity hub/tavy woodlands just outside tavistock (on the cornwall side) has a few other bits here like high level treetop stuff for the kids etc
River dart country park has a newish (a couple years ago) very small little bike park with a few jump trails, if your not into jumping (which im not) then give it a miss, its normally full of kids and gets boring quickly if your not repeating a jump trying to perfect it !
Scadson freeride park/scadson woods around torquay, never been but always intended to check it out ?
Theres a few more dotted around but thats got to be enough to keep you busy this summer !!