You could actually get away with that in Spanish by saying '¡Qué bebé más feliz!', as neither that noun nor adjective change with gender. Thanks for the idea, Alex!

The options are limited, though. You could also have 'triste' if crying. Or 'grande' if big. Adjectives that end in '-ente' also work, but I guess 'inteligente' would be a bit previous, and 'sorprendente' rather rude, for example!

Drafts are also considered to be the kiss of death in Spain - close all windows!