I say thank you the next time I see/am in contact with someone, but wouldn't write a letter especially.
My mum however writes a letter at the drop of a hat and does thank people for cards.
We do 'unbirthdays' for Gchildren and have done for children until they left home. Traditionally the 'unbirthday' gift is a tube of Smarties!
Locked room for breakables when grandkids stay?
Daughter in law from hell, mini videos on Facebook…


. Each to her own.
... or thirds! 