To complete the passage correctly, the verbs are as follows:
The cows in the field ate the grass, and the bird on the fence sang. The children were on their way to school looked at the bird. The stream flowed among the trees quickly.
So the completed passage is:
The cows in the field ate the grass, and the bird on the fence sang. The children were on their way to school looked at the bird. The stream flowed among the trees quickly.