Liz and Michael are comedians well-known in the worst stand-up spots of London. With years of depression, dozens of psychedelic trips, and zero kids they are well equipped to educate the audience on the intricate details of parenting.

They are miserable and not getting any younger. All their friends seem to be having gigs, so naturally they conceived a gig together as well.
Liz did it out of need for someone helpless, disoriented, and who loves her because they don't have anywhere else to go.
Sure, the audience might get traumatized, but they will grow up, earn their own money and seek psychological help.
Michael just wanted to show his wife that he can be spontaneous. He has no wife now.

Come if you want to be gaslighted, patronized and to produce laughter as an unhealthy defense mechanism. In other words, come if you too had an "interesting" childhood.