If you would like to create a group where the guardians respond on behalf of their child, you need to create a Children/Youth group:
Create the group and select "Children/Youth" as the age group (you can also change this within the group settings at a later time)
When adding the member, add the child as the member without a contact point, and the guardians below with their unique contact points. (see example below) It's important to not use the same contact point on more than one person and only the one it belongs to. Read more. It is sufficient to add 1 contact point on the guardian/s for them to receive the group invite.
If the child is also going to respond to events, you can chose to add their contact point as well. If they are not going to respond, or don't have a phone number or email dont and a contact point next to their name.
If you are adding siblings, you need to add them separately, the same way as other members. You can add several members with the same guardians. (Do not add the guardians contact point on the child, only on the guardian)
When the child is old enough to respond themselves, you can add their contact point as well. Then both the child and the guardians will be able to respond. Read more.