I'd allow two hours unless you ask them to bring items faster. Maybe with a 3 year old, that might be better if your meal is later in the evening.
We took our DS in January 2016 at age 3.5, and again in December 2016 at age 4.5, and the meal was too long for him. First time, he got super cranky so we skipped the dessert. Second time, he fell asleep on the bench seat. Both times, we were seated within 15-25 min of our ADR which wasn't bad considering how late I've heard it could be. It just took a while for them to bring all the food around.
Once you're done, it's easy to get to the beach from 'Ohana...just go to the first floor of the GCH and exit out the back or side doors; head in the direction of the lagoon and you'll find the beach.