I agree with Wallin, having spawned 6 different species of Bristlenose I've found BN usually won't eat their own fry once they leave the cave but sometimes they will when they are young parents, and other will most certainly will. Not always, but it's pretty common. BN will also eat the eggs if they are young parents and get spooked (keep your flash-lighting and other cave checking options to a minimum).
I would suggest an in-tank fry saver. Once you notice the fry starting to leave the cave, put the fry in the container (or the whole cave if it'll fit as they are about impossible to get out if dad is still in there). Keep them there until they are ready to leave (4+ weeks, longer if your saver is large enough). Then they will have a much better chance of surviving. Fry saver makes it a whole heck of a lot easier to count them, keep your eye on them, feed, clean, etc. To clean, get a piece of flexible airline type hose and use that as your 'gravel cleaner'. You can just suck up the poo off the bottom of the saver. You can control the water flow by squeezing the airline tube with the hand you are holding the draining end, slowing down the flow when fry get close, or if you suck one up (you can drain him right back into the saver)
Good luck!