On a short flight like that, you're really mainly looking to board early enough to secure four aisle or window seats. Don't sweat getting eight perfect seats: just four pairs of seats together.

Checking in 24 hours ahead of time will be sufficient. You do not need to print anything in order to check in. You can check everyone in from one device - I've checked in my whole family for the return piece of a vacation using only my mobile phone.

Print all of your confirmation numbers at home before the trip. (Or know that you can access them from your mobile device.) The confirmation number plus last name is the key needed to OLCI.

You can reprint boarding passes using the kiosks at the airport.

You don't need to buy-up your fare. You don't need to buy Early Bird. Those are needed to sit at the front of the plane, but getting pairs of seats at the back is not a problem as long as you check in at the 24 hour point. You'll likely have high A or low B boarding passes.

I imagine a gate agent would bristle at 7 older pax accompanying one five-year-old during family boarding. Might depend on the ages of the next-youngest children. If you've checked in at the 24 hour mark, it really shouldn't be an issue either way.