Attach a large, tiered birthday cake -- either precut or self-drawn -- to the bulletin board. Decorate the cake with "frosting" and decorations, just like a real cake. Make a candle from construction paper for each child with his name and birthday on it. Place these candles on the cake. When it's a child's birthday, move her candle to the top of the cake and make an announcement about the birthday girl. Do this with weekly or monthly birthdays if doing it daily is a challenge.
Write or print out in large letters the song, "Happy Birthday." Make the letters large enough that the preschoolers can see them. Break up the song phonetically to help the children read the words. Leave the name section of the song blank. Have each child write out his name on a precut piece of paper. Write the name of the child in small letters under her interpretation of her name, so you know who is who. Write the birthday of the child on the paper as well. Place these pieces of paper in a pocket labeled with something like "Birthday Stars." Pull out a child's name when it's his birthday and place it in the blank name spot of the song. Have everyone sing to her.
Create a birthday party scene with a birthday cake, party games and a set of 12 balloons. Label each of these balloons with a month of the year. To make it easier for the preschoolers to understand, put the months in order, by having January at the top of the bunch of the balloons and December at the bottom of the bunch. Write each child's name in the corresponding balloon to his birthday. Also, write the day the child was born next to her name. Point out the child's name when it's his birthday to the rest of the class.