Is Sundrop also a child in mind? Do they have this sort of relationship? I got the impression that it's rather harmless PG-13 hippie stuff like trading flowers, singing kum-ba-yah and smoking pot. (figuratively speaking)
I don't really remember all details of his arc, though, because it's so, to put it kindly, "fleeting" in S10.
When he made the moves on Olivia while in his child body, that was creepy (luckily for Olivia too).
But well, even if it's this sort of relationship, it's not "creepier" than Willow doing the nasty with a snake or Buffy boinking dead corpses. It's not the first time that they took controversial subjects like rape, bestiality, necrophilia, even pedophilia and many others, then twisted it into something relatively "normal" or for comic relief and called it metaphors.
I think it's no big deal compared to all this other stuff.