We have been many places with our boys, now 11 & 5.
DR was my FAVORITE vacation EVER!!!!!!! Everyone is sooo nice over there & helpful. Beautiful beaches & water. We went end of Dec - mid January. Weather was awesome - no a/c, but no need for it.(We rented a rather large house w/in laws & other family). I felt very safe the entire time.
Was a little difficult at times becuase we don't speak spanish=), but brought a dictionary. They eat LOTS of beans & rice made their way.
You MUST go to the waterfalls. You climb up these falls & walk and walk. BUT, then you come back & get to slide down some of them and dive or jump off others(you may climb back down at the end also). It was SO beautiful, just bring good water/walking type shoes! I would love to get your email address & I can send you some personal photos, if you like. LOTS of tours, great, but shop around for the best deals!! Everything can be negotiated.