I've eaten dinner there three times, once fairly early, around 5ish, once more around more typical dinner time of 6-7ish, and once late, I think it was 8:30 PS, and all three times they had some type of children's activities. My dd loved the coconut races and the hula lessons.
They rotate through the activities all night so you will most likely get to do everything at least once during your meal. Sometimes they are a little slow getting started though.
My kids have ordered typical kids fare there - chicken nuggets, grilled cheese, hot dog etc. They still ate some of the regular dinner like the salad and wings and dessert.
I think it's best to have ADR's for any sit down meal you really want to go to. This is just what I've gleaned since starting to research our 1st trip, we've got all ours lined up (including O'hanas)
A PS (Priority Seating) is not required for O'hana, yet highly suggested (for almost any sit down restaurant). It can be a hit and miss though...when I was there last summer, we had to wait for about 30 minutes after our PS to be seated...annoying, yet highly worth it!!!