Since my kids were not capable of writing a thank you at age 3, I vote for you writing a note from both of them to the gift giver. And age 1, my kids understood that the appropriate thing to say was "thank you" when handed something, but they definetly didn't understand the concept of the word, so I wouldn't expect a heartfelt thank you from a 1 yr old but would expect the parent to thank me for remembering their child.