There is nothing wrong with things being on-topic on more than one Stack Exchange site. There are plenty of examples of overlap between Stack Exchange sites, such as the overlap between Science Fiction and Fantasy and Movies and TV. There is no inherent problem with this, and no reason why this would be a bad thing.
Let's let Parenting do their thing, and we'll do our thing. Parenting will make decisions about their scope based on what is good for parenting, and we'll make decisions about our scope based on what is good for us. If that means that there are questions that can be asked on both sites, then so be it.