Maybe your children just were not sure how to react to him because he does not have a specific role in their lives. Nothing is cut and dry with children. He is just the man their mum is dating and someone they see however often. Too distant to be a friend but not close enough to warrant a parental figure role in their lives. Odd place to be. As long as they are being respectful and at least cordial, just leave it suspended in midair. Some things are best left alone. Maybe once they are older or more used to him, they will become friends. 12 year old's are funny little beings. I dread the pre-teen and teen years.
It sounds like the wrinkles have been ironed out for now, and I am glad you guys enjoyed your weekend.