Okay, the strip is the beachy standard Spanish resort bit. It's the club that has the sports facilities etc.
It's pretty quiet at night IME. El Rancho is quite central so you might get a bit more noise from being near the shops etc, but in general it is quite sedate.
There's a spa at the Hyatt, there's a (very) small beach on site. Presumably you'll hire a car, in which case you can drive out to some of the better beaches (10 mins). You're not far from Murcia and Cartagena for sightseeing. There's a watersports centre not very far outside (windsurfing etc). At Cabo de Palos there are places to eat. Lots of walking to be done too. If your DCs are of footballing age there is a football academy for children.
It's split into areas and think that you are tied to the pool of wherever you are staying, although there is a big public pool next to the tennis club which has a very good shallow area for small children. You can also pay to use the indoor pool at the Hyatt spa (by the Las Lomas apartments).
I haven't flown to Murcia in a while because BA stopped flying there and I recall the remaining flights were at odd times. These days I use Monarch to Alicante which is a little over an hour from LMC. Never had any problem booking flights and if you're planning this far ahead you'll be fine! By the way, for car rental I really like La Manga Car rental. They have so far always managed to prove me with age appropriate and clean car seats, which puts them streets ahead of everyone else, and because they are not one of the bigger companies there is often no queue to get your car at the airport. They're at Murcia, Alicante and inside the club.
HTH