I went to HDDR when I was 4 and my sister was 2 and my parents contend it was the best exp we had at Disney that entire week - they took loads of pictures of us dancing along and having a ball. I would not think twice about taking my preschoolers to it - DH and I went last month again (his first, my third) and we had a blast as well, it's so active, with nonstop entertainment, and lots of audience involvement.
we love Ohanas too, though the two places are hard to compare - Ohanas has some children activities, yes, but the majority of the time you are just dining. It also takes a while to eat there, without constant entertainment, unlike HDDR that has constant entertainment and things going on.
I don't think you can go wrong with either - have a great time!