There is a baby shower cake here on this site, the person made a onsie out of fondant it is beautiful. Maybe you can do the same only this time make a template of a shirt and pants. Maybe something simple pink pajamas with white poka dots ( did I say that right? poka?) anyway.
Hey you could even try doing the buttercream frozen transfer that way you can get more detailed on the pajamas
(I am all for the frozen transfer, I am working on my son's guitar frozen transfer ( ice cream cake) Wish me luck!! Hope this helps! Don't forget to post your cake when your done, I would love to see what you come up with.