I love to include the birthday boy in the thank you notes. They are both too young to write them themselves so they usually are included in the decoration.
For the first birthdy parties for both boys I let them go crazy with finger paint on plain 8.5 x 11 paper. When their masterpieces dry I cut out squars or rectangles (depending on the size of the notecard) and and attach the drawings to the front of the notecard. I can then write and sign the inside of the card and the gift giver gets a thank you note from me and a personal drawing from the birthday boy.