If the parents don't think it matters to acknowledge Christmas and birthday presents that you have spent time choosing, paid for, spent time wrapping and packing to post and paid, probably through the nose, to send, it is hardly surprising that it doesn't occur to the children themselves. It is extremely rude and, furthermore, it is thoughtless because we know that sometimes parcels go astray so you have no way of knowing if they have arrived safely. I think you should at least mention this last point to the parents and suggest that it would be nice if the recipient(s) could speak to you on the phone or send you a note or picture, depending on age, to let you know about their celebrations.
I, too, have five grandchildren and for years sent expensive parcels across the world for birthdays and Christmas. I always had a phone call the day of the birthday/Christmas Day in spite of a huge time difference – to thank me and chat about what other presents they had received, the lovely cakes their mummy had made and so on. Indeed, it was quite an effort to get them stop.
I think it would be better to tell the parents how you feel – calmly and politely, of course – rather than simply to stop sending presents. Explain how much you would like to hear if they have played with toys or if the clothes fit – from the recipients themselves.