I can answer the question but it's vague, so it would help to know more information. If the ex is not following the court order regarding picking up the children on the scheduled days, your daughter is the person who has to do something about it, if at all. As the grandparent, you can't do anything about it even if you'd like to. The only one who can do something is your daughter. If he's not picking the kids up at all, you can't force him to do so. If the custody order has been made by the court a long time ago and if she does not have full custody, then she may try to change the custody to sole custody if he's not interested in picking up the kids.
This can be answered in so many different ways because there are so many different scenarios that could be happening, and the question is not detailed enough for me to know exactly what is happening, so I'll give you as many different scenarios as I can.
If he's picking the kids up on the wrong day, and insisting on the wrong day, then she can bring this back into court because he's not complying with the order. It would have to be a serious violation, though, as in it has to be continuing. If he's continuing, let's say, to cause trouble about picking up the kids, she can possibly do a motion for contempt.
If you have a copy of the order, that would help, but even if you don't, I just need to know what he's doing in violation of the order. However he is violating the order, she can discuss with her attorney and possibly bring him back to court.
Much as you want to help her, she has to do this herself. As the grandparent, you don't have standing to bring this back into court, but she can.
Does this answer your question? If you give me more information I can help you further.