There are very few free spectacles in Vegas anymore that are actually family friendly.
This is one of them.
The best time to go is on an early summer week night when it's around 70 degrees and the lights are even more spectacular.
Ever since the TI Pirate show turned into "Sirens of Seduction" the fountains have become more popular.
The problem is that people are trashy and they throw coins and trash and food into the fountains. Perfect.
Do yourself a favor and book an al fresco dinner at one of the many restaurants near the fountains.