http://android.joao.jp/2013/09/back-stack-with-nested-fragments-back.html

gistfile1.java

    @Override
    public void onBackPressed() {
        if (!returnBackStackImmediate(getSupportFragmentManager())) {
            super.onBackPressed();
        }
    }

    // HACK: propagate back button press to child fragments.
    // This might not work properly when you have multiple fragments adding multiple children to the backstack.
    // (in our case, only one child fragments adds fragments to the backstack, so we're fine with this)
    private boolean returnBackStackImmediate(FragmentManager fm) {
        List<Fragment> fragments = fm.getFragments();
        if (fragments != null && fragments.size() > 0) {
            for (Fragment fragment : fragments) {
                if (fragment.getChildFragmentManager().getBackStackEntryCount() > 0) {
                    if (fragment.getChildFragmentManager().popBackStackImmediate()) {
                        return true;
                    } else {
                        return returnBackStackImmediate(fragment.getChildFragmentManager());
                    }
                }
            }
        }
        return false;
    }