Our rule is the children get to open all gifts and play with everything on the special day. That night everything goes into a bag in the closet until thank you notes are written. I promise you that they will get done, someday. Our oldest turned 6 in May and just finished his notes in December*.
Keep a folder of all the thank you notes your children receive so they have inspiration to write their own. They love getting mail and when they re-read the notes they received it helps them to sit down and send notes to thank others.
Simple wording for early writers:
Thank you Aunt Sue. I like the books. Love, Austin
Dear Aunt Sue,
Thank you for the books. I enjoy reading them at bedtime.
For our 5 year old, she dictates what she wants to write (the first example is hers) and we write a template without names or gifts and she copies and fills in the blanks.
Our 6 year old has free reigns and you can tell by reading his notes that he spells everything himself. He does a great job and is very sincere.
*I have to tell all that he didn't fully realize what the rule was until he saw his sister writing her thank you notes in December and playing with all her toys. It took him 5 minutes to finish all his notes. He wrote one round of notes directly after his party but didn't write them for gifts that came in the mail. Poor thing actually forgot about all his other gifts. He wrote all of his Christmas notes on the 26th of December.