I haven't heard of 1900 PF having the same issues with wait times substantially past one's ADR time, but there is no guarantee at any restaurant that something won't happen to make them run late.
In terms of 'Ohana, I have had better luck in terms of being seated close to my ADR time when I had an early reservation. I think 'Ohana opens somewhere around 5:00 pm. It's too early to coincide with the EWP, but if eating at 'Ohana is important, that's something to consider.
As aaarcher86 said, 'Ohana is open for breakfast (character) and dinner (no characters) only.
In terms of the EWP, you can watch it from pretty much any of the monorail resorts or the MK depending on the timing. It's nothing spectacular, but it's fun to watch. Could you have an early dinner at 'Ohana then go look at the monorail resorts or the MK and catch the EWP from one of them later? You could ride the monorail or take the ferries from the various resorts (not quite sure what activities would interest your family).