It’s completely up to you although obviously it’s likely to get covered in icing and so I don’t recommend anything that you aren’t prepared to say goodbye to if the stains won’t come out (that’s particularly the case where the icing is a dark colour).  Some people prefer their little ones to be full dressed (although I do recommend bare feet), some wear just trousers or a pettiskirt, or some smash in just a nappy although a nappy cover can look a bit nicer.  There are lots of specially designed cake smash outfits available now that they have become so popular so auction sites, Facebook marketplace and Etsy can have some beautiful things.   Unfortunately, I don’t let you borrow my outfits from the studio for the actual cake smash as they would quickly get ruined so just bring along something that you love.