Throwaway for obvious reasons and on mobile, etc. I need some advice.

Yesterday, I asked SS 5&6 how there week was at home with Mom. Usually they like to tell me all about what they did and who they played with. This week they were very shut off. They each said "We did nothing and saw nobody". I thought it strange but thought maybe they were just not wanting to talk to me. They often don't at the suggestion of HCBM who will blatantly call me a whore and use other colorful language when talking to their Dad in front of me, and who knows what she is saying to them directly about me.

Their mother had surgery recently. Ill let you use your imagination but it was a very expensive elective surgery and as she had asked my SO for extra money to help with school supplies because she was broke, we were not made aware of the surgery until the result was obvious when she dropped them off. BM has trouble finding childcare typically with the exception of one individual who DCF has deemed a danger to the children and they are not to be left alone with them. We would have gladly taken them extra had we known about the surgery.

After the shock wore off, their father asked them this morning who watched them during mommy's surgery. 5M began to say something and 6M started telling him to be quiet and to not say anything. They were asked who told them not to say anything and the response was "I'm just scared I don't want to be taken away" followed by lots of tears. Not wanting to stress them out more, we dropped the subject.

Im at a loss on what to do here. I dont know the whole story about why they can't be with this person, and other than suspicions, I don't know that they were. Their father is trying hard to make things civil with BM and doesn't want to unnecessarily cause an uproar. Regardless of what is actually happening at home, I feel that the children being told to keep secrets is a form of emotional abuse. DCF is already involved from a past incident. Is this something I should report? Should I just hang tight and see if something comes of it? I care about the boys and their well being but I dont know if it's a complete overstep to do something. Is this one of those situations I should stay out of and let bio parents handle?