Unless you know the baby's sex, I would use both boy and girl gingerbread shapes. Make a fan fold of a large piece of brown paper and cut out a gingerbread person chain like the old style paper doll chains to festoon the ceiling. Add eyes and buttons with colored markers or paint. Use real ginger people to make a circle on the cake top. Put them so that a candle can burn in between each one if you like.
Whenever I need pictures to decorate for a theme, I search for the pictures I want on the internet and print them out in color. Here are some gingerbread people I found. If you do a search you will find whatever you want.
Use the cutters you would use for shapes when you make real gingerbread boys. Trace around the edges and cut them out and glue them to poster board. You could dip them in paint (I forgot what the process is called) and glob the outline onto your poster board. You could then "dress" the gingerbread people in boy and girl attire with a little paint and creativity.
Just use paper streamers and balloons for decorations. Not every thing needs to have a theme. Otherwise if you can find gingerbread shaped decorations go for it. You could even draw gingerbread shapes on poster board. You would have to use craft paints to paint them, but it is a solution. Drawing the gingerbread shape would be easy to do.
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