I've been looking into how to do this and have found quite a nice way so long has your willing to use some private apis.

The issue is as WorstNoob states that there is no way to show only the children for a tree model using a QSortFilterProxyModel (that I could find), however you can use an internal Qt model called QQmlTreeModelToTableModel (QtQmlModels/private/qqmltreemodeltotablemodel_p_p.h) which as the name suggests proxies a tree model and converts it into a table model. Now you can proxy this table model and filter out parent rows as they are technically no longer parents. This is the model that Qts QML TreeView uses internally for what its worth but it should work whatever your usecase. It also preserves child parent relationships albeit in a different way so you still have that info available should you need it. One gotcha to look out for is that you need to manually expand the rows, otherwise it will only show the first row in the model.