Is everyone missing that you’ve planned 5 accommodations over 8 nights? This is one sure fire way to ruin the trip. It’ll become all about check in times, packing and unpacking.
Key West is a slog for 1 measly night and why break up Miami into 2 one night stays? Presumably you’ll collect and return your hire car to the airport too so you’ve the traffic and drives in and out of Miami doubled if you do 2 stays.
Flamingo to Key West is a 4 hour drive without stopping!
A week is so short, especially with a long haul flight either side, but since you’ve booked it now I’d say,
2 nights Miami Beach. There are lots of tours that will take you around the sights (little Havana, wynwood walls, celebrity mansions). An evening boat trip will be cool too. The art deco hotels are all very instagrammable for the teens.
Leave Miami early on the last day and head to Flamingo (if you’re not bothered about the non-resident premiums!) Then drive to Islamorada or Key Largo for 2 nights. Accepting this will be a long day but committing to that will save you a lot of hassle.
You can enjoy the upper keys, snorkel at John Pennekamp state park and relax after the first busy few days.
Then head down to Key West for 4 nights. You won’t regret having longer there I’m sure!
You could break up the drive back to the airport somewhere if you really wanted but personally I don’t see why you would want to. It just eats into your time with such a short break.