These tips apply mainly to 5U-8U games.
Dividing a team between two fields could be done in many different ways. The two leading ways are to divide them randomly while the other is to have more aggressive kids on one field and less aggressive kids on the other. Discuss with your opposing coach at the beginning of the game but you don't both need to use the same method.
Here's one example of a way to split kids "randomly" between two fields. Have the kids line-up numerically by their jersey number, typically this would result in them being ordered smallest to largest. The smallest number would go to field 1, the next smallest numberto field 2, repeat. When substitutions are necessary, the player that arrived latest sit out first. The next player to sit out is the jersey number numerically larger; start over with the smallest jersey number after the largest number. Using the jersey numbers sequentially makes it easier on the coach.