Are thank you notes absolutely necessary or is a phone call from my kids to Grandma good enough?

Kevin says:  The rule for kids is the same as for adults: A heartfelt verbal thank you for a gift received in person is sufficient. If a gift is not given in person (delivered by mail, for instance), a written thank you note is mandatory. As soon as children are old enough to print their names, they can at least sign a thank you note written by (or dictated to) mom or dad. I know many parents who shelve a new gift until a child has written a thank you note. No gift should ever go unacknowledged. Ever.

Beth says:  If you want to go by the etiquette books a written thank you is a must.  However, my mom really enjoys a conversation with my kids as much as a note in the mail, so often they just call her to thank her and chat.  To me, what is a must, is that the gift be acknowledged promptly.  Many times I have sent gifts and have had to say "Did he get my gift?" to the parent. Unacceptable.  If I took the time to buy and send a gift, your kid can take five minutes to call and say thank you.  That said, I do written thank yous as well.  I had both my kids write A Gay a thank you note after he took them on their 12th Birthday Trips of a  Lifetime and there are other situations where I have insisted on a formal reply as well (especially to relatives that my kids might not feel comfortable calling).  In a perfect world, it would be all thank you notes, all the time.  In my world, I just try to get the gift acknowledged right away.