Do not ruin your child/rens grandchild/rens christmas this year or any year just because your ex gets to see them this year.
It will not make you feel any better, no good can come from it!!!
There is no benefit to anyone from destroying what should be a special day for your child or grandchild.
Try to do something useful and helpful for a change, volunteer your services at an old peoples home , soup kitchen or help homeless people for the day.
You have probably spent many hours, days, months or even years feeling bitter and vengeful and wasted so much time screwing yourself and children up.
Do yourself a favour and have a day off, your child will thank you for it.
It will be the best Christmas present ever you could give to your child/grandchild and to yourself.
You may even enjoy yourself for a change and realise that you can be a better person.