If you fall out with your inlaws over this your parents win
This ^^^
Your PILs are most probably trying to minimise the fall-out of what has happened for your sake and the sake of your children, rather than because they have any liking for your parents and their nasty shenanigans.
Probably the card was just signed "Fred and Alice" (or whatever their names are) with no message - it was an acknowledgement that they had received a card, rather than genuine Christmas wishes. They may even have had a pile of cards they had received and just automatically written out responses without thinking until it was too late.
I can see how it would have hurt you, though - I would have felt the same way myself. You must feel betrayed by people that you have trusted enough to share a very unpleasant experience with, and it is hurtful. However, as you have encouraged your PILs to acknowledge your parents in public, they most probably aren't aware how painful you have found your own parents actions.
I don't think you are being unreasonable - your feelings are obviously very raw and this is very painful for you - but I think that most people who haven't been through anything similar won't be able to feel how very painful and paralysing it is. You may appear unreasonable to them because they just can't imagine the depth of feelings involved.
Try not to take it out on your PILs - explain to them (or get your DH to) how upsetting the whole thing is for you. Or if you can - tell them the whole sorry story which may help them to understand. It might be better if you could leave any revelations or expressions of upset until after Christmas, though, or until you can talk to them without getting too upset. They will probably be horrified at how much this is hurting you and it would be a shame to hurt them, too, when they don't know what they've done that is so soul-destroying to you.
If you feel that you can't go, perhaps you could claim to be unwell or something so that it doesn't look as though you are being "precious" about it. (And it's not a lie, either, because this has obviously knocked you sick.)
I wonder why your PILs and DH thought to mention it to you? It could have been so that your parents couldn't crow over you about it - it would really have caught you on the hop if you mam or dad had tried to rub your nose in it (which my be what they are attempting to do).
Whatever you decide to do, I hope that you can enjoy Christmas with your family and put your poisonous parents where they belong - in the dustbin of your life.