Jeez. Some other threads have been having pretty reasonable discussions. Maybe getting kids involved pushes the wrong buttons for a lot of people.
Anyway. I'll break with precedent here, OP, and take your post at face value. Sure, a healthy eight-year-old's getting under 1:50 is certainly doable for most, without any kind of hardcore "training." If she can get 2-3 days per week of running, she'll be fine.
A 5-10min "run around the block"--assuming she can complete it, which she will almost immediately, if she can't already--is fine. One of those a week works. (NOTE: If she's building up to this, it's much better that she alternate a run with walking, rather than shifting to a "survival shuffle.")
The thing she should get from that run is that she can run a pretty long way, so she doesn't have to worry about *lasting out* a 400--as long as she doesn't start the 400 like a sprint! She probably just has to work on getting the feel for a speed between a jog (her round-the-block pace) and a sprint (which is probably what she does in soccer).
If a track is readily available, just do a few 100's with her at that "medium" speed--around 24-27 seconds. Get a nice walk between reps, or do other stuff (throw a ball, play leapfrog, whatever) between. She's mostly building a skill here, so she should be pretty fresh for each 100. (If you have 100m available elsewhere, e.g. a soccer field, that'll work, too. Or, if *you* have a good feel for the proper speed, just run with her for 25-30sec at a time.)
She'll probably be able to handle 4-6 100's with no problem. Next time, you can extend the distance a bit, but subtract a rep. Example:
6 x 100, then 5 x 150, then 4 x 200, then 3 x 250, then 2 x 300. [If she can handle 2 x 300 (as much rest as she wants) at the right pace, she'll be fine on Test Day.] Just have her occasionally do a rep by herself. This will show her that she can pick the right pace all on her own.
Then, for the test, she just needs to pick that pace again. If the other kids want to start at a sprint, let 'em--she'll pass some before the finish.