At the youth level, the point of being on the field is to gain some sense of teamwork, learn about competition, exercise, gain a measure of confidence and self worth, make some buddies, and to have a good time . Don't ruin it for the youngsters by making them do fitness. So, with this is mind, tattoo it in your brain that soccer for kids is supposed to be fun. No matter what you think or feel about fitness, straight fitness, in general, is not fun. The only time it becomes fun is when the player has determined and committed him or herself to the game and desires to play at a higher or the highest level possible and the benefits of the fitness outweigh the pain of the moment.
Kids just know they are running a lot and want to stop.
Straight fitness for them has no meaning besides it's boring, time consuming, and probably, painful. It'll drive them away . . . There's no reason for it until a later age. Now, that's not to say I don't think they should work hard, but it has to remain in the context of the game. Make them work hard with a ball or in an exercise.