Dr Karl (aussie science guy) got all the well-wishers to put their unsanitized finger into his eldest son's mouth on his birth day. Sample size 1, worked OK.
Yeah, but a baby has transferred IGG through the placenta, and IGA though milk, so for the first six months they are more protected from severe disease (at least for quite a few pathogens their mother has recently encountered). After that, exposing someone to pathogens, especially immune evasive ones like herpesvirus, gets a lot iffier. It's probably safer to only expose a baby/child to their mother's pathogens, as well as various food allergens to try to prevent food allergies from developing.