Do kids need to write them?
It is never to early to start teaching your children good etiquette.
If they are too young to write, they can draw pictures and you can add a thank you note at the bottom.
There are pre-printed cards that make it easy for new writers. The note is written and your child just fills in the blank with names and the gift they received. (Most stationery stores.)
Talk to your child about why it is important to acknowledge others when they take the time to give a gift. When it comes via mail, it is nice for the person to know that you received it.
Notes don’t have to be long, just sincere.