We've done 5 night Bahamas twice, 3 night Bahamas, 6 night Bermuda, and 7 night Mediterranean. 5 night Bahamas is our favorite itinerary.
It's the least "adventurous" itinerary, you just plop yourself on a pool/beach chair for 5 days and relax. 3 or 4 nights is too short to get comfortable with the ship in my opinion, and 7 nights is a bigger commitment of time and money.
My kids HATE the kids clubs (10, 7, and 5 now, we've been sailing since they were 7, 4, and 1) and cruising is still their favorite thing in the world. This is going to be very unpopular for me to say, but the kids in the clubs behave like maniacs and my kids don't have the mental patience for all that hyperactivity. They'd much rather spend time in the lounges doing family crafts or hanging out on the pool deck, and we like being all together. The only times they go to the clubs are for scheduled activities like slime-making or on our last day when Mom and Dad need to keep them occupied while we pack.
The only change I would make to your plan if I were to make what I consider the most-perfect first cruise would be to do the Magic first, as the smaller ships are extremely easy to digest and the larger ships slightly more overwhelming. But I think the Magic is in Galveston next spring and my preference isn't so strong that the ship should override the dates and itinerary you prefer.
Stay on the ship in Nassau. Nassau sucks.