Group conversation is usually done among friends. Each group is set up by one contact person who becomes a group manager.
When the group is started, it will only be invited by default. This allows you to join a member only if the current member invited the group, but it does not necessarily know other members in the group. They can see messages from strangers, and vice versa. A group administrator can make a group public. In other words, anyone can search and join a group.
If you are group chatting with a stranger you do not know and are uncomfortable it is advisable to inform your child that you should leave the group and discuss.
Users can leave the group at any time. If your child is an administrator of a group, you can delete or prohibit unnecessary users.
Also, if you are not an administrator, you can ask the person in charge. Administrators are the only users who can delete or prohibit members of a group.
Read more about how to report a user on kik here
Deleting or prohibiting members of a group will prevent you from sending messages to that group.
However, users deleted from the group can be re-added by group members, but prohibited users can not rejoin the group unless prohibited by group administrator.