In terms of his rights, it will depend on the type of order that he has from the court.
A child arrangements order sets out with whom a child lives - the is effectively 'custody'.
A prohibited steps order prevents another person (usually the other parent) from removing the children from the main carer's care without their permission - if he has this, then he may be able to seek the children return by contacting the police.
If there is a dispute and she has taken the children from his care, social services may also intervene if they are notified.
If no-one feels the need to remove the children from the mother's care urgently (for example if they are deemed to be 'safe' and 'well') then your father in law may have to apply to court himself to enforce the court order in place that states the children live with him.
So there are a few options depending on the legal framework in place.
Perhaps if you can find out a little more information about that, I can be more specific. For now though, I hope that this assists?