Since he's already experienced pretty much everything else in act one (belly of the whale, meeting the shadow self, descent into the Abyss, etc), I'd say that Childe is at least approaching the midpoint.

Following the Liyue archon quest, he's been "wandering" so to speak, on his own personal journey, similar to the Traveler. He's been everywhere except Sumeru (which I have my own personal theory about) so far, and there's only one region left between our current one and Snezhnaya. Act two will certainly kick into gear in Snezhnaya, and that will be...huge. And that's an understatement. There will be some incredibly significant reveals.