Do it with breaks.
Go at a medium speed (for her): not sprinting, not racing, but also not a shuffling jog-and-slog, which an athletic kid most likely won't enjoy. So maybe 8-10min/mile pace?--whatever pace has her actually running.
Take a break from this running BEFORE she "has to," and do something else: some kind of calisthenics (vary it from break to break), or just a walking break with neck/arm/shoulder mobility stuff, or even a different kind of running: backward, carioca, slide step sideways, etc. You want her to be fairly fresh during her actual running, so you're using the breaks to *prevent* fatigue, not to pay an oxygen debt.
[An eight-year-old who's already 4'10"/147cm is either maturing very early or is going to be very tall as an adult. Either way, she's going to need whole-body development to stay healthy, which is why a variety of variety of exercise, including core and upper-body stuff, is important to include with the running.]
At the end of the run, definitely let her "sprint home" if she wants. This is natural for a kid and she'll be well warmed up by then. Then give her a minute or two of easy walking to get her breath back, rather than just stopping dead.