The topo map below provides some perspective on the local terrain. According to KSL, the vehicles had stopped in Maxwell Canyon, which extends northwest from the thumbnail and were swept into Short Creek, the main drainage that runs past Hildale and ultimately southwest to Arizona highway 389 south of the map. KUTV reports that the cars were swept away near the crossing of Canyon Street and Williams Avenue. I can't find the latter on Google Maps, but the former is near the thumbnail.
The time of the flood is a bit uncertain. The KSL report says a bit before 5 PM MDT (2300 UTC). Media reports last night suggested a bit after 4 PM (2200 UTC).
The radar loop below shows two periods of heavy rainfall that may have contributed to the flash flooding (the Hildale area is identified with a brown box). The first sweeps over the area from about 2000–2030 UTC (2:00-2:30 PM MDT), the second from about 2215–2300 UTC (4:15–5:00 PM MDT). There is a possibility that the two storms provided a one-two blow that resulted in a more substantial flash flood event than if either had occurred in isolation.
Radar estimated precipitation shows a swath of rainfall ≥ 1.25" Maxwell Canyon and Short Creek areas through 5 PM. Greater values, exceeding 2 inches are found to the southwest. Note that there are uncertainties that can affect both the amount and location of the radar precipitation estimates, so further analysis will be needed to gain a better estimate of how much precipitation fell in the drainages that generated the flash flooding. Analysis is also needed to ascertain the importance of precipitation intensity on the flooding of this event. The totals below occurred primarily in two bursts rather than steadily over a multihour period.
An unfortunate characteristic of the geology of southern Utah is that the precipitation produced by monsoon convection can be concentrated into severe flash flooding that strikes quickly. Flood waters can overtop or destroy banks along flood channels rapidly. This serves as a tragic reminder of the importance of staying well clear of washes, slot canyons, and flood waters when heavy precipitation is in the area.
Addendum @11:10 AM:
The NWS storm report issued this morning (below) suggests that flooding produced by the first cell produced the fatalities. Note the 3:18 PM event time.
Thus the timeline of events is still uncertain. It could be the fatalities occurred with flooding from the first cell, with the flooding exacerbated by the 2nd cell (which would have further complicated rescue and recovery efforts). Careful analysis of the timeline will be needed to nail the details down.