Is there music linked to this 'running on' event? If not, is the director willing to put some music to it? Something a la 'Sabre Dance' or such? In either case, it might be easier to represent the slo-mo with the music. I have a pitch bender utility in my audio software that I would use to accomplish this.
What are the circumstances that are behind this 'running on'? Panic, happiness,
etc? If the music idea wouldn't work, you could try recording the kids either cheering with excitement, yelling in panic or anger, whatever the reasoning, and then slow that down.
If music is permissible, another option that might be easier is a tempo shift. With the running (fast), start with an upbeat or frenetic tune. Sabre Dance could work for this, or some great fast tempo march. Then, at a point in the music that the kids could listen for, switch to an extremely slow song. Count Basie's 'Lil Darlin' could work, or any good slow instrumental that would allow the kids to really do the slow mo. Give it a bit of music to establish, then work in a speed-up selection, or find something that starts with a cymbal crash and resume fast speed.
A few options, let me know what might work.
PM me if you want.