Since I posted this, I found that that either mechanical switches or piezoelectric with ipx5,6 or 7 ratings failed when exposed to rain, sun and Florida’s high humidity. I therefore ended up using a garage door type sensor. It in turn operates a relay starting both OS devices. The sensor is mounted only 3 to 4 inches apart. I had to block almost all the transmitter output and receiver input with electrical tape. Only a very small slit of light is allowed. When one puts a hand between the sensor a relay activates starting the OS. In your case the second relay is not needed. They are “xin-sheng 1 pair Waterproof Single Infrared Beam sensor Photoelectric Infrared Barrier Detector” from Amazon. This is very inexpensive and will work with 24V AC.
Using city water sounds expensive for a splash pad if it’s used a lot. Programming the OS was the easiest part. Don’t expect the standard sprinkler valves to create the pop effect of some commercial splash pads. We use the same sprinkler valves and the water comes up reasonably fast, but no pop.