What is the law for a father taking his children to his house, if the mother says no... in the UK?

He can certainly apply to the court and the court will make an order which appears to be best in the children's interests. That is the focus in care proceedings; it's not a question of which parent is entitled to what so much as a question of what will most benefit the children. They, after all, are entitled to spend time with their father and only if there is evidence of their not being properly treated or being exposed to bad influences will the court refuse him access. However, they are very young still and perhaps it isn't appropriate for them to be away from their mother at this stage.