Sadly, we don't have any grandchildren to spoil on birthdays and Christmas, but we do have 2 sets of children that we buy for. The response from each set is completely different.
Family 1 - next door neighbours, 2 children, 8 and 5. We send cards on birthdays, and presents at Christmas. Never a thank you from the children, although the parents do say thanks when I deliver the parcels before Christmas.
Family 2 - dad is colleague of my husband, 2 children , 14 and 12. We send presents for birthdays and Christmas, and always get a hand written thank you.
When they were babies, Mum would write the thank you card, and baby would do some squiggles with a crayon. They were teaching them early! As the years have gone by they progressed to writing their own name, and then later, their own letters, often on hand made cards. We have loved seeing the progression of their handwriting! We don't see them in person very often, so usually there is a photo in with the thank you card.
I'm seriously considering stopping the presents for next door, as we don't know if the children have liked any of their gifts over the years. Maybe just send a card to the family?
My children always wrote thank you letters for presents. I remember trying to keep note of which gift came from which auntie at Christmas, so it could be mentioned in the letter!
Take care everyone xxx