I'm guessing you're talking about Waimea Valley. I was going to respond with the same thing! We took my (then) 3-year old there last year. It was great. While there are paths that aren't paved, the main path up to the waterfall is paved and very doable with a stroller.
There is a train that goes through Ko Olina. My son enjoyed it. Ko Olina was probably the prettiest place it goes through, and you obviously are already seeing the area because that's where you're staying, but if you have a train fan it's a great option!
http://www.hawaiianrailway.com