If you are buying at Terrazas you will need a car to go anywhere. There are not many facilities nearby, apart from the Industrial Area where there are some bars, restaurants and I believe there is a bowling alley there, but not 100% sure as never used it! There is a lovely beach down at Los Narejos, and also there is a long stretch of restaurants across the road from the Narejos hotel. There is also a nice beach at Los Molinos. We use the following to find places to visit, as the reviews are by people who have been to visit the places: http://www.tripadvisor.co.uk/Attractions-g187518-Activities-Murcia.html
There is a zoo and Water Park called Terra Natura about 20 minutes from Terrazas (by car) which a lot of people say is good. We have never been so can't comment. It is quite expensive though at 25 euros per person (think they do a family ticket) but you can spend all day there.
We don't have children that age (ours are grown up!) so am not sure if this is the right place for them. It is very quiet, and other than the pools on site and the few playgrounds, there is not much else for them. There is a huge shopping centre in San Javier called Dos Mares, and upstairs they have a bowling alley and lots of restaurants. There is also a small fair with rides and things for children at San Pedro del Pinatar and this guide tells you all the beaches near to Murcia. http://murciatoday.com/introduction-to-beaches-of-san-pedro-del-pinatar._9769-a.html#.U7BCW2cU_IU
I hope this will help, good luck with your decision, it is not an easy one!