[Pretty Butterfly]

You can get the cops to go with you.

Since he has some time, he can also go to social services ( they probably already have someone, from the court case etc, that the kids have met and talked to maybe) and let them know that she has said she wont hand them over. They can talk to her beforehand, and be there at hand-over instead of the police.
If she realises he WILL fight to see the kids, she just may back down anyway.