R.D.
Is there a legitimate reason he doesn't like her? Is she inappropriate around the children? Unless there is a good reason for him to not want the children around her (uses drugs etc.) then I would tell him that she is family and you want her there. At the very least tell him him that you will be going to take the kids to see her at a certain day and time and he can come with you or stay home-It's his decision.
Good Luck and Merry Christmas!
P.S. I totally agree with everything that Mimi says as well. She put it very well.