Google Calendar

1. Open calendar.google.com on a desktop browser (mobile can't import).
2. In the left sidebar, click the + next to "Other calendars" → Create new calendar. Name it "Custody" and save.
3. Click the + again → Import. Upload the custody-schedule.ics you downloaded, and pick your new "Custody" calendar as the target.
4. Hover over the new calendar → click the three dots → Settings and sharing → Share with specific people. Add the co-parent's email.

Apple Calendar

1. Open Calendar on Mac → File → New Calendar. Name it "Custody".
2. File → Import, select custody-schedule.ics, and choose the "Custody" calendar as the destination.
3. Right-click the "Custody" calendar in the sidebar → Share Calendar. If it's an iCloud calendar, the co-parent will get an invite.