H.S.
sittercity.com - Go there and search your area.
I'm sorry for your situation of having to start over. The hours you need sound good for a college age girl who has classes through the day, or maybe even retired person who can be available only 2-3 days a week.
I live in Ohio, and I'm assuming Vegas is certainly quite different, but $50 a day sounds a little high. The rates in the midwest must be lower. But we are talking twins here, twice the work. You're likely to find care from someone who would ask for less. $40 or so for each evening. I'm not sure. Good luck!