I wouldn't try to write "Happy Birthday" on a sponge cake that you won't be icing, but if you want to do it in icing, buy a tube of premade decorator icing. It's sold in the baking aisle near the candy cake decorations and comes in a few colors; it's vanilla flavored, but I always think it tastes funny. Kids like it, though.
Another option is to get a "Happy Birthday" pick - these will be located near the party supplies in places like Target, or by the greeting cards & birthday candles in the grocery story.
I would actually recommend the pick over the icing, as it's easier and looks a bit more impressive than the writing.
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Liz
DD (3/2010)
"Make mistakes! Get messy!" - Miss Frizzle