Parents who date: has this ever happened to you? How to explain it?

I think the best thing to do in this situation is explain to her that Kelly is in fact Daddy's girlfriend. I know that will leave behind a tricky situation to explain if the relationship doesn't work out, but it's done now and I think it's the lesser of two evils, the alternative being Tess believing all friends behave like this potentially raising some serious questions amongst other adults if she tries to reenact the behaviour with her friends.