We took our DD to Vegas for a family reunion when she was almost 4. We had a great time and she was oblivious to billboards etc. We stayed at the Flamingo which has a wonderful pool complex. During the day mostly we were at the pool. We'd usually go out early in the morning to walk around and get breakfast (Krispy Kreme at the Excalibur was a favorite). We'd go out again in the early evening (the Belliago fountains were a big hit). We stuck to DD's normal 8pm bedtime so we were safely in our hotel room before to real action got started.
Our 12 yo nephew was also part of our group and we were all more concerned about what he'd see then we were about the little ones.
BTW, we were there for 5 nights.