In physics, it says time is defined by it's measurement.
If time is the word, I'll use a clock to define it.
Clock; "a device other than a watch for indicating or measuring time commonly by means of hands moving on a dialect"
If we use the common idea of a clock with hands on a face, it measures from 1 to 2 to 3 to 4 or whatever...
1 to 2, can be described as A to B.... it just measures a change.
A clock displays this change, defining time.
This simple A to B seems a satisfactory way of describing time, to me.
The person asking this question wanted the simplest definition of time, not the most clear in the eyes of a child.
If they wanted the simplest definition of the word "naked mole rat" I would give them;
"A warm-blooded vertebrate animal of a class" aka mammal
rather than "A rodent (Rattus and other genera, family Muridae) that resembles a large mouse" aka rat.
So seeing that this question was posted in the Physics section, I defined time according to the realm of physics, leaving no room for any controversial definitions, and concluded that A to B would seem an accurate, simple way of defining time.