This situation sounds very suspicious and it's way too serious for amateur guesswork. I think that counseling is needed ASAP to get professional advice on it. In the meantime, since you've got three children in the house, he needs to go. There's really no excuse or reason to take a chance, this is some guy you've known for a year~ your kids are what matters.
I do not think you are overreacting..As a mom reading your post something is not quite right. Since you are a mom as well, what does your
I'm with you on the whole thing not smelling right, but we don't know for sure it's not perfectly fine, that he really does identify himself in a father role. So I would caution you to make any accusations. The key here is to focus on yourself. YOU are uncomfortable with it rather than HE is wrong. Think about it. These girls have mothers and fathers who apparently trust your SO to take them to a concert, apparently overnight. If the parents trust him, and he is choosing it, you have no say in any of that. You do have a major say in what you will tolerate, so focus on that. Tell him you do not feel comfortable, focus on how this affecting you, what you want and what you will not accept. Even just saying "I want to understand this" can help, as long as you are making it about what you want and need rather than trying to tell him what he should be thinking/doing. But also recognize he has a choice to make here, and those girls and those girls parents are also making their choices, and none of that is up to you. If you make it clear that you are uncomfortable, concerned, and will not accept being with a man who does xyz, and he ignores your discomfort, dismisses your concern and does xyz, then you have answered an important question. XYZ is more important to him than you or your relationship. If you value him and the relationship, and he values xyz, then it's just not going to work.
The problem is going to be views his role as a fatherly one, then fathers can take their daughters to concerts and it's not a problem. Usually SOs who make their partners choose between the relationship and the kids, lose because most parents choose the kids. It might help to be very clear that you support father/daughter activities between the two of them, you simply want to be involved and included. If he offers again to take you with them to the concert, you might want to consider accepting so you can see first hand what is going on.