If the gifts were opened in person and your son said thank you to the givers, you are not required to send a thank you note. However, that being said, now's a great time to get your son in the habit! If he received anything in the mail from relatives he should write them a note. Even if it's just a crayon scribble in a blank note card on which you've written a quick thank you, the gift givers will cherish it. Or, consider sending a Polaroid picture of him holding the toy with a big smile along with his scribbles. If he is old enough to dictate, write for him what he has to say. If he can write, let him! Kids make Great thank you notes! If grandma loves those thank you notes, she'll be sure to send more great presents for his future birthdays! You don't have to send anything to the other guests, but if there was a mom or two that hung around during the party and helped out, send them a note to thank them for their help. Kid parties are crazy and chaotic times at best and any mom that lingers to help deserves to have her effort noticed!