I always have my daughter send out thank you notes for the gifts she receives both for her birthday and for Christmas, even if she's already said thank you after she's opened the present.
I feel giving thanks and having gratitude for the things we receive is a dying art these days. There is absolutely no reason why children cannot be taught how to give people the proper thanks for a gift they've received.
My sister also makes her daughter send out thank yous. My brother and his wife, however, have never instilled the art of thanks in their three kids. In fact, I was floored when I received a thank-you note from their oldest child, thanking us for the Confirmation gift we'd given him. I thought, all hope isn't lost yet with this one...
As far as the upcoming friend's birthday--is the friend celebrating and having a party, or do you mean just being cognizant of the fact that said friend has a birthday so that she mentions it to the friend/says Happy Birthday to her at school or something? Yes, if your son or daughter is too young to watch the calendar/know that his/her best friend is having a birthday today, I would remind her/him so that if your daughter/son wanted to say something to his/her friend, then he/she could say something.