Digital time is inherently superior. Analog time requires the extra cognitive step of translating to the numbers being represented. Digital time skips that process and shows you the numbers directly.
But on this point there isn’t anything inherently more abstract about Arabic numerals as a representation of numbers than the angles of the hands on a clock face (ie a short hand to the right is 3 and upright is 12, etc is a pretty efficient way to convey a number). As for what can be read quicker probably has overwhelmingly a lot to do what was learned in youth. Similar to stenographic shorthand this can probably be acquired but there just isn’t much incentive to do so.
There isn’t necessarily any extra “cognitive step” in the pattern recognition of a clock face vs that of a numeral.
I don’t know much about research on this particular area but there is some in the related area of written language.
If they can understand how the hour hand goes around, they can understand how a fuel gauge goes from Full to Empty. They can tell how a battery gauge on their iPad means they're running out of power.
Kids that age are soaking up concepts.