Can react hidden cards on battlefields i control during my beginning phase?

It depends — you can only do it if something during that Beginning Phase puts an item on the chain. Starting your turn or entering the Beginning Phase does not itself give you priority, and Priority is the only thing that lets you play a card (Rule 312.2, 335) — so you cannot simply flip a hidden card faceup "at the start of my turn."

If a beginning-of-phase trigger fires (yours or a battlefield's, e.g. "at the start of your turn..."), that ability goes on the chain as a pending item, which creates a Closed State and gives you a window to play a Reaction. Since a card that's been hidden gains Reaction from the turn after it was hidden (Rule 811.6), you can play it there — again only from the second turn onward, never on the same turn you hid it.

One thing that doesn't change: the hidden card must be played to the battlefield it was hidden at, and any targets must come from that battlefield (Rule 811.1.d). If no trigger fires in that Beginning Phase, just play it in your Main Phase instead, where you get priority normally.