this is a bit of a tedious read (sorry jona), and probably not immediately obvious how to apply in this case, but it’s probably our most authoritative english resource on は vs が on the forums (I think this was the original source)
this is also quite a deep dive
exactly. and yeah, “i have a child” is probably a more natural interpretation…
it’s ok, they’re talking about existence. if they wanted a child they’d use something like ほしい instead. and in that case they’re probably saying they want to make/adopt a child 
that aside, 居る (to exist) is the only common ichidan いる so in context with conjugation, it’s still not likely to cause confusion. not that I’ve every heard 要る being used unless it’s negated, and 入る is only really ever used as part of compounds ayway