I think that depending on age of the children, it's actually about a half-time job.

This puts you in an awkward position - it is really hard to get a half-time job, or at least a good half-time job - even if you can find one, you have almost certainly killed long-term career prospects because employers generally prefer full-term employees. And there is no point screwing up both of your careers by both going slightly part-time, so you easily end up in a rather awkward situation of either outsourcing a lot of parenting work that isn't ideally outsourced, or one of you giving up their career.

It is usually the person earning less who makes that sacrifice, so even a fairly small income disparity turns into a huge one.