Do you write thank you notes for Christmas presents from your relatives?

My sister-in-law has been writing thank you notes for my niece and nephew (they’re too young to write them themselves) since they were born for Christmas and birthday presents that we gave them. My family growing up only wrote thank you notes to friends and just said thank you to our relatives when we opened the gift. Now that I have a son of my own, do you think it’s rude for him not to write thank you notes for Christmas and birthday presents from aunts in uncles and grandparents? Especially if he’s not writing them himself?