My mother was previously married to a man, and had 3 sons with him, then divorced, remarried and had me. The first guy (my brothers' father, but not my stepfather or anything like that) was the one who abused me during summer visits. My situation is kinda different, but this is what helped me.
Validation
Just having someone acknowledge how it made me feel lifted a huge weight. It didn't matter if someone could understand perfectly why I felt that way, but just having someone acknowledge my feelings, and saying that I deserved to feel that way helped so much. Even if you don't agree with whatever they are feeling/say at least tell them you acknowledge it. It helped me feel safe when talking about what happened or my feelings, I wasn't as scared to open up, I didn't feel like I was going to be judged.
Talking
There were times when something would keep running through my head and just saying it out loud helped to stop the repeating cycle. This one could be hard, especially if it was about a specific trauma. I always left the option open that someone did not have to listen to me, if it was going to be too hard for them to handle. I didn't want to make things difficult for others as well. I respected their limits/boundaries.
Hugs
Lots of hugs, but only when I asked for them, or just gave them. Touch by someone who loves me and cares about me can be very calming when things are hard. When I went to the police station I was death gripping my carer's hand. That physical contact helped so much, it was a good touch, and reminded me that good touch is possible. Having him just sitting next to me for moral support ment so much. I think you'd be surprised how much that could help them during the trial. Just having you around can mean so much.
Love
Just tell them you love them. What happened to them really sucks, but you didn't wish it upon them, you did not do this too them. You may feel guilty for not knowing or not being able to prevent it, and you can tell them that, but if they say it's not your fault, or that you shouldn't feel guilty then listen to them. I had problems with blaming my family for what happened, but I realized they couldn't change anything now, but they could support me and help me make things better. Focus on that. Now when I tell them to stop feeling guilty they know I truely mean it. I don't want anyone else to feel bad for what he did to me. It wasn't my fault and it wasn't their fault either, it is only his fault.