OpenFL-Bitfive isn’t actually part of OpenFL but a 3rd party HTML5 backend created by http://yal.cc/
You should post an issue on their github or contact them for an answer as it looks like they simply haven’t implemented this function. It shouldn’t be too hard to implement the function yourself if you wanted to fork and add it - it could just involve swapping items in the children array. You could then send a pull request and have your changes merged into their repository.
If you are using a framework that enables another backend by default, you can also compile using -Dno-custom-backend in order to disable it and try OpenFL standard