This is a wonderful day hike but I would suggest looping back to your car, instead of going to the valley.
Do not be surprised at how taxing this hike can be. It sure sound good that it is all downhill, but the last 3.5 miles down the Yosemite Falls trail is brutal. After having done this exact hike as a two night backpack trip with my little ones, I would suggest looping back to your car at the Porcupine Creek TH. The map says 6 miles or so with decent elevation gain, but this would be equally as taxing as the hike down to the valley and there would not be the hassle of an hour (or more) bus ride back to your car.
When planning for my trip, I looked into the bus ride too. I quickly discovered it would be a huge pain in the ass, so we left a car in the valley and and one at the trail head. We had to collect the other car after the hike, but it was cheaper and faster and a lot less stressful to do it this way. Even with all this, and a highly successful adventure, we all agreed we would have rather looped back to the car.
Don't get me wrong, the Yosemite falls trail is cool and there are some neat views along the way, but not as many as you think. It's just a whole lot of head down, trudging down stairs.
Just my 2 cents worth....
"It is all very beautiful and magical here - a quality which cannot be described. You have to live it and breath it., let the sun bake it into you" - Ansel Adams