"Oh ho~! Well, I'm rooting for you, Estinien. Don't be a disappointment," comes Childe's sing-song voice echoing a bit in the throne room, and he's standing a distance away, but gives a cheeky wave.

Really, he is absolutely rooting for him— he wants to see what the guy who he seems to share some common things with has to offer. It's just a guess on his part, however, but he imagines clashing with Estinien, even injured, would take a majority of the time they have until that spell completes. That would leave less time to fight with everyone else, so he's going to let everyone else deal with Estinien. If he's still standing towards the end, maybe Childe will engage him, then.

"I'm afraid I'm going to have to go over you, though, to find myself an opponent. I hope you don't mind—since you simply can't protect everyone here."