Link Siblings on Registration Forms

When a parent selects the 'Add a Child' option on a registration form, they are given the ability to submit information for all their children at once. The registrations then appear separate within the Leads section (one registration per child). However, once the forms are submitted, the children are not linked as siblings. It is currently a manual process to link them within Leads or after they are enrolled. Client would like the link as siblings to happen automatically when parents are submitting them on the same registration form.