I actually added a reset button to those default "get next" actions a short while ago while working on the DialogManager for my game, without even knowing that there seems to be a demand for that functionality.
As it turns out, there couldn't be anything more trivial: since those actions rely on caching the current index in a private variable which don't get reset when re-entering the action, so you have to do it yourself at the start of OnEnter(), like so:
if(reset.Value) nextItemIndex = 0; reset.Value = false;
(as you can see I even added that the reset bool automatically gets set back to false after resetting the index, since there's no plausible reason to keep resetting the action, and if there is, one could just keep the bool set to true in the action after that "get next" action).
You can find the two customized actions for 'Array Get Next' and 'Get Next Child' in the attachments and I would also urge HutongGames to add that option to the standard actions, so that no one has to rely on these or their own custom actions just for that function (even though you rarely need it, it's very nice-to-have if one does).