In what way did the wife say the children were abused? Did she say they were beaten or sexually abused? You couldn't tell by just having them for a couple of weekends. Their wounds do heal of it's physical abuse -- and if it's sexual abuse, they're certainly not going to TELL you!
So how do you know it isn't true? Did you ask them? You can't tell by looking them over. Abusers are very clever and they don't leave many signs. About the only way you're going to find out is to ask. But you have to be careful how you word it too. You don't want to scare the kids, and also children TELL on adults! They will tell their Mom and Dad what you asked them. So be careful how you word it.
If they tell you they HAVE been abused, let their mother know ASAP and let her make the decision as to what to do.
I wish you the very best. Take care and God bless. Hugs, Lee