Answer:
A group of parents were supervising the children.
Explanation:
If you want to change a passive-voice sentence to active voice, find the agent in a "by the" phrase, or consider carefully who or what is performing the action expressed in the verb. Make that agent the subject of the sentence, and change the verb accordingly.
For example
The car was driven by Paul's mother
Ask the by who/wat question. This identifies Paul's mother as the object of the sentence
The subject is "The car"
The verb is "was driven"
The object of the sentence is "Paul's mother"
Switch the object and the subject, adjusting the verb accordingly.
The active voice becomes Paul's mother drove the car
Paul's mother becomes the subject and drove becomes the verb
In the sentence
The children were being supervised by a group of parents
Subject: "The children"
Verb: "were being supervised"
Object: "a group of parents" (answer to by who? question)
So the active voice is
A group of parents were supervising the children
