Do you know if they saw the pics accidentally or on purpose? It honestly sounds like they saw them by accident while looking through his phone.
If it was an accident, I would say something to him about not letting such things be around the kids. If it was on purpose, then I would tell your attorney and seek supervised visits.
As long as they are in no immediate harm, he has a right to see them as their father. If you try to prevent those visits without having proof of his purposeful sexual indiscretions and the court on your side, I think you run the risk of it backfiring on you eventually. Did he show them the pics or did they come across them on their own?