I am wondering the same thing.
If I pulled an NJ, it would be nothing. But that doesn't seem right.
For her birthday, kids 'bought' stuff for her, and I had them make home-made cards for her. For my birthday, I got a cheap present from her, nothing from the kiddos, not even a card.
I think they should honor their mother, but RL friends have been telling me to forget about it, she doesn't deserve it since it won't be reciprocated. But the point is not that it isn't reciprocated, it's that the kids should focus on Mom on that day, and at their ages (10 and 6) they need a prompt from someone (me).
Would you like your ex to help your child with Fathers Day?
I would, but don't expect it. It goes to furthering the relationship of the child with the other parent.