This drives me crazy as well! I have my daughter write a hand written note for everything that she receives. I understand that some people feel like an e-mail, call, or even text is enough of an effort. I am fine with that, but no acknowledgment whatsoever is unacceptable to me.
I have many nieces and nephews. We did stop sending things to the ones who never acknowledged. We didn't plan that way, and it wasn't an angry spiteful move. But they never said boo about anything we sent, even if we spoke to them on the phone. But when they graduated high school, you bet they found a stamp and our address to send an announcement, expecting a gift. That's when we decided NO MORE. We send a card, but that's it.
Now for you, these are kids and grandkids, so I don't know. First, I would speak to your children, and let them know that you plan to stop sending gifts to those who don't acknolwedge. And let them know if/that you are ok with a quick text or call even. I would be fine if my parents said this to me, it would straighten me up and realize I hadn't been grateful. You wouldn't be telling them what to do, you'd be telling them what you plan to do.
The other option is to send an unsigned check, or an empty gift box- I'll bet they find a way to contact you then! :-)
Seriously, just tell your kids. I know some people say they send gifts because they want to, not because they expect anything back, but I think that's ridiculous. Who sends things only because they expect something back? Silliness. It's basic manners and gratitude. Period.