The hike to Dewey Point is pretty popular and the trail is generally easy to follow. The first half of the hike is pretty easy, but the last half of the hike can feel strenuous since the trail winds a bit and there is more up and down. It's a little over 7 mile round trip. If it has not snowed in a while, then the path will be very clear and a bit easier to hike. If there is fresh now and you are the first to hike it, then watch for the trail markers in the trees. They are triangle shaped (with the trail number) or rectangular flags attached to the trees. When you get to the point, it will be obvious where the drop offs are. The view is amazing and well worth the effort. Just keep in mind that 7 miles in snow shoes is different that 7 miles on a trail, especially with the young ones. I took my son on this trip when he was 12 and he did pretty well, but then he has been backpacking with me since he was six. Here's a write up I did for Backpacker Magazine:
http://www.backpacker.com/rip-go-dewey- ... ions/13824. Make sure you download the pdf file which gives more details. Here's my write up for my blog on the hike. As far as how long it will take, that all depends on you. When snowshoeing, I always estimate I'll travel about 1.5 to 2 MPH (depending on the photo opportunities). Figure on how much time you will want to spend at the point too. The rocks at the point are a great place to eat lunch! I did the trip again last February and It was still magnificent, but it seemed a bit more challenging than the first time. Mostly because the snow was a bit fresher.
If you feel you are not up for the 7 miles, another good hike is down to Tuoulmne Grove. It is a shorter hike, about 2 miles downhill to the grove and then back up. I hiked past the grove a bit, but you don't have to. I did that trek in heavy snow and almost blizzard conditions and loved it! Here's a write up I did for that trip.
http://www.trimbleoutdoors.com/ViewTrip/352465Good luck on your trip. I just returned from two days in the valley and it was magnificent!