My ds has suffered from night terrors and nightmares for most of his 8 years. This reached a head when he started getting separation anxiety. I couldn't go into another room without him following me and he panicked if he couldn't see me. He wouldn't go to birthday parties alone and whilst out he would never ever leave my side. He would always have his hand on me.
I sought advice from Young Minds who have a helpline staffed by experience and trained people who work with children. The advice I got was amazing.
The basic rule I found was that instead of discouraging ds to get out of bed at night, I should tell him that he could come and get us at anytime of the night for anything and that he would never be in trouble for doing so. This gave him the reasurrance he needed that he could call on us for help whereas previously he was anxious about disturbing as naturally, after the 4th or 5th time we were not impressed. This alone made a huge difference at night.
I was also able to talk to him about his fears. His were monsters so we drew a picture of the monster and I put a clown's hat on him, a moustache etc and encouraged ds to do the same. Before long we were both laughing at the monster which took some of the fear away from him.
Never tell them that they should stop being scared or that they are just being silly. Take their fears seriously and listen to them. Let them know that you understand and perhaps tell them what you were afraid of when you were a child.
Communicate with them at night about what tomorrow will bring as my ds used to worry if things were not planned. So we'd take some time to talk about how the day went, any worries he might have and what we would be doing tomorrow so that I knew when he went to bed that he didn't have any anxieties bottles up.
Worry dolls may help and also a box to them to keep nightmares in. They draw a picture or write down what they are scared of and put it in a box where it can't harm them. The next day you can both burn it.
I would thoroughly recommend Young Minds for brilliant advice and all I can say, having dealt with similar myself, is to keep reasurring them of your love, make sure you praise them lots (parenting is often too full of telling them what they can't do, what they have done wrong, etc and we miss the opportunities to praise good behaviour, even playing quietly for a time should be praised) and take time out to listen to them and chat about their fears. It takes a while but it does get better 