You know, I really enjoyed the trailer, but my wife and I concur on this.

No. It’s not Supergirl who has the problem. ‘Superman’ is not called ‘Superboy’ because we do not refer to grown men as ‘boys’ as a general rule. At least, not in the UK. You might, fairly rarely, say ‘Are you going out with the boys’ in reference to a group of men going out with a (usually) male friend, although ‘lads’ is probably more common, but outside that one example, I can’t think of another where it’s reasonably common.

Women though, apparently it’s okay not to differentiate between an adult and a child.

Meh.