Sorry to hear your boys are having a hard time...the posters are right, it could be a number of different things, but trust your instincts, as they are usually right on the money. If you have concerns of abuse, you must act quickly to get them help so they are safe.
It could also depends on how they have been raised, either home, and is this NEW behavior for them or has bed time always been hard? Has there been significant family structure changes happening in either household that may be increasing their fears? Or changes in bedtime routine maybe?
Our child was diagnosed w/ Generalized Anxiety and the psych said she had an inordinate amount of fears, more than we realized she worried about. She sees a play therapist, and she helps her deal w/ these worries, through play sessions. Our daughter has been known to make situations advantageous to her and also to "stretch" the truth to get what she wants, which is to have an adult sleep with her.
If you have concerns, and wonder a good way to broach the subject with the kids about what is wrong, a good way for us has been to bring it up as a general topic, while doing something else like it isn't a big deal (even though it is) or say something like, "when I was little I hated to sleep in my own bed...or use a special stuffed toy, or even a real animal to relate those fears onto, like, "Teddybear told me he wants to sleep on the couch or with me, do you think the (kids) bedroom is too cold for him (or too dark) or whatever". It seemed to help our child be able to talk about her fears with being less vulnerable.
And let them know you don't think anything bad of them for their fears, that everyone is afraid of something(they may worry over that inside) The biggest thing is to give them a feeling of safety and security, which helps our child.
I pray it isn't child abuse, their custodial home doesn't sound like a good home structure for the children and sounds like Social services should become involved. Document everything they say or you observe of their behaviors, the professional help you get will be asking for that.