I have neither time nor help to count laps. How can I keep track of children's mileage?

Measure the distance around your track or field. Have each child run twice around, for example. Some will finish sooner but all will have run or walked the same distance. When recording mileage simply multiply the course/track distance by the number of days run; enter the same mileage for all participants. Or, have the kids run (run/walk) for 15 minutes and record a mile for each.