For me first click opening submenu and second navigating to parent menu page feels really natural on touch devices (its a little of “if at first you don’t succeed try again approach) but if adding a big toggle +/- button is becoming more popular these days I guess that would work just as well. Perhaps leave both as an option?
Actually, I’ve done a combination of both and I think it works really well – i.e. first tap activates the sub menu, second tap activates the parent item link and in addition a big button is available for toggling the sub menu at any time. So my plan is to stick to this as a default from the next release. There is still some work left on other features but it is coming soon I hope.