Hmm i dont have any kids i know about and so maybe shouldnt comment. I also didnt have any parents either really so im probably out of my depth. I understand that you wish to see your children and my belief is that if your children wish to see you then no one should stop that happening.
I am aware that the legal route always seems like hell and seldom resolves the matter in the best interest of anyone. I imagine all of us male or female have exes that we have had hard times with, and a human condition is often to wish to hurt or punish someone we perceive has wronged us in some way.
Taking the legal route just seems like a perpetuation of a fight that can go on for years.
I have no idea why your ex is witholding access to your children and i have no idea whether that is reasonable or not. If i was in your shoes i would try and avoid the legal route. I would try and make peace with the ex rather than continuing a fight through the courts which seems to do nothing other than making people more stubborn and unreasonable. If her problem is with your character that has led you to prison then perhaps showing her you are a reformed character and a loving, caring, reasonable father and a good role model might help. Obviously this will require great strength and patience on your part. Best of luck