Am I reading the itinerary correctly, that you're only spending half a day in Lisbon? It's a fantastic city, and probably deserves at least a couple full days.
A note on Sintra...it's a great day trip, but if your parents have trouble walking, it may not be the best. You can take the train out from Lisbon, which lets you off right next to the bus that circles around to about half the castles/sights. However, at each one, there's a lot of walking involved, and still a lot of hills. I remember the walk to and inside the Moorish castle, in particular, as extensive. I don't believe renting a car would be any better than taking the bus around, although it may save you a bit of money with 4 people. I mentioned the bus only because that's what I did, so I'm not really familiar with the parking there or if that would require even more walking for your parents.