To add a child waiver, first make sure you have configured Age groups by visiting Settings / Age Groups.

With Age groups set up click Settings > Waivers > + Create New

When creating a new waiver from Show At, select Add Connection from the dropdown and complete the rest of the waiver as needed.

Next, select Applies to, and choose the relevant age group:

Whenever a member adds a new family member and say for example they are under 16 they will be presented with the waiver for under 16's to sign.