I think it falls into the same category as giving someone a gift of TY cards or an etiquette book. I think it says, "You are rude," but without the courtesy of saying those words directly. And I don't think that's a good way to go about things. (Of course there are always situations where people like getting etiquette books or pretty TY notes, but we aren't talking about that here.)
I think it would be better to talk to the parents or, if old enough, the grandchildren directly, in whatever manner works best. Or, just stop sending the gifts. I did that once--I was tired of getting no acknowledgment for my gifts, so I just stopped giving them (though I did not say anything about it first). And no one ever said a word, and as far as I know our relationship wasn't affected (I didn't see them much anyway).
For me the key is to stop the resentment. You can't make people acknowledge your gifts--even if you send them a blank TY card, even if you ask them directly to respond. But you can change your own actions/thoughts--resign yourself to being okay without acknowledgement, or just stop sending things. Generally speaking no one will die if they don't get a birthday gift from Grandma.
Though, if odds-wise you think there's a good chance your child or grandchild will notice the loss and say something, I might preemptively mention sending a TY--I'd rather the conversation be, "Well, I asked for a TY and you didn't send one, so no more gifts," than "You should've just known to send a TY and you didn't, so no more gifts." I think the latter is an accurate point, but if I'm about to stop sending gifts to my grandchild, I'd rather give a warning first.
Reading over my post, it all sounds a little dictatorial... I figure, no one has to get gifts, and no one has to send TY notes in response. So if sending a TY note is too onerous for you, that's cool, I just won't send a gift. Then hopefully we will both be back at a happy state, as opposed to me waiting resentfully for my TY note, and you staring guiltily at the gift. Also, I personally am very broad about what I consider an "acceptable" TY for a gift--email, text, phone call, tell me in person (if we meet soon after). I don't demand calligraphy on parchment delivered by an owl or anything. But if it is seriously too much trouble to message me on Facebook saying, "Hey, thanks, can't wait to use it!" then let's end this awkward dance and depart the field as friends, you know?