This is a difficult one as I have a good relationship with my daughter and gd, and always get a phone call or a thank you letter. I'd be very hurt if any gift was returned to me saying it wasn't liked. A text message just with "thanx" written is unacceptable, and would also hurt me. Does the OP normally have a good relationship with her grandchildren or not as she doesn't say? In my opinion I'd not stop sending gifts to them, but maybe ask them if there is anything they would like in particular, or just buy a multi store voucher for them in future so they can choose their own. If the OP stops giving anything then that could easily trigger a family argument. However at the same time I can totally see how one would think why bother at all. No appreciation, so what's the point. I brought my kids up to always show gratitude even if they didn't like the gift, as it's the thought that counts. These days parenting is very different though. I believe in moving with the times, but I still have morals, and it costs nothing to be polite and say thank you. I'm thankful my daughter and SIL have taught my gd to have good manners.
I do hope the OP manages to resolve her problem without causing a family rift.