YANBU.
I always remember my Xs birthday/Christmas/Father's Day and get the DC to make a card and present for him (e.g. hand-painted 'dad' cup). I don't want my DC picking up the awkwardness nor do I want my X to be handed a tool he can use to manipulate the DC.
However, I will NOT buy anything, even a token £1 present. He pays no maintenance so I'll be damned if I'll spend any of my hard-earned money on him instead of the DC.
This is not reciprocated. I'm not bothered for myself (I'd actually feel a bit awkward if he got a gift) but it annoys me that he doesn't bother with a card for the DC's birthdays ("because they can't read so what's the point") and while he does get presents, they are either OTT wonderful or completely inappropriate and rarely come wrapped ("because they're only going to rip the paper off").
I insist my DC make cards/presents etc because it's not about whether the X deserves a present, it's about social niceties, learning how to keep relationships functioning smoothly and taking the time to remember little but important facts about people. It will make them better human beings.
That said, as my DC get older, I will not make them continue to make or buy cards/presents for their father if they choose to stop doing it, though I will remind them if they appear to have forgotten. As older children I suspect they will see his lack of interest for what it is and reciprocate in kind, which I certainly won't encourage but neither will I guilt trip them for their father's failings. You reap what you sow.
If my DC forgot an important occasion in my life and obviously felt bad about it, I would be at pains to point out that it's ok so that they didn't feel bad (depending on their age, of course, as I think showing hurt feelings is entirely appropriate once they are in their teens for example).
I think there are quite a lot of non-resident parents out there who put in very little emotional, practical or financial investment with their children (let alone remembering the resident parent's birthday), yet expect all the upsides of parenting. It frustrates me that "being the bigger person" often translates as "the mother has to suck it up because otherwise she's hurting her DC" yet rarely seems to apply to the father. I feel that point should be made, but I'm NOT saying that it applies to the OP at all. In her case I think she's being entirely reasonable. She has clearly tried to be understanding and is aware of the exceptions to this rule as she's pointed them out herself. And she's right - it is sad for the children caught up in it. whatdoyouthink - as the DC get older, they'll draw their own conclusions about who's behaved most in the children's interests, and while it must be frustrating right now, these things tend to come full circle. Just try to reassure them for now.