Hi Tara, I don't know your kids and only you know best, but I do know for our DD at those ages, that would definitely have been too late. I think the biggest challenge you will have is that you are wanting to provide a great experience that, because of the late time, might not end up BEING a great experience and so is self defeating.
Again, this is not saying there are children who couldn't handle it, but waiting until 8.30 PM with our DD (especially if it were after a day at a park) would have been pointless.
That said, I also don't know the reason specifically for Ohana. Is one of their favorite characters there? Is it their favorite food? Did one of them ask for it or is it something you think they would like?
My advice would be to keep trying for an earlier ADR but, again, only you will know best.