You might want to ask what type of pool heater they have. Most use a heat exchanger, which pulls heat from the surrounding air. These don't really heat the pool much at night when it's colder (and not at all if they outside temp falls below 55 degrees).
Some homes however, have gas (propane or natural gas) powered heaters, or electric pool heaters, and these are capable of heating the pool 24 hours a day in any temperature. They also cost a lot more to run, which is why most people don't have them, but some do, you should ask!
With either one though, a pool blanket helps tremendously. Not all homes have them, so you should definitely ask.
A pool blanket for a 12 x 12 splash pool costs like $75 by the way, so they are not that expensive at all, no reason not to have one. Not having one actually would only drive the heating / energy bill way up, they pay for themselves in 1-2 months.