You need the following items:
European Health Insurance Card (EHIC). You need one per family member and this will cover you for emergency treatment in Spain. You can only use this card on a temporary basis until you take up residency or a maximum of 3 months.
E106. You can get this from the social security department in UK. It will transfer the money you have paid into the NHS and will only cover you for a maximum of 2 years. This will entitle you to free medical treatment at a GP in Spain and reduced cost prescriptions. Murcia, unlike the Valencia region, does not provide free medical treatment for people under retirement age (as you mentioned you were taking kids with you, I assume you are not yet 65) who are not working. You will get reduced costs prescriptions for ongoing problems (like diabetes) but will have to pay for the medical treatment.
If you intend to work in Spain, you will go onto the Spanish NHS system and receive free or reduced cost medical treatment.
At the council elections in January, the parties promised to provide free medical care for all those living in Murcia province, just as they do in Valencia and Catolonia.
You need to check out medical insurance before you go.
Dental costs in Spain are not provided on the state health service except for children up to 15 which only covers checkup, fillings and minor dental work. Everyone else pays but the cost are about half those in UK.
To sum up, you need to look at your situation. Obviously, under retirement age. Get a 106 and EHIC to cover you at the outset.
Under 65 and intend to work? You will get on the state health system once you start paying the equivalent of National Insurance. (I understand you can declare as self-employed and pay the National insurance contributions of around 240 euros a month. Have a look at previous threads)
Under 65 and not working? You will have to pay so look at health insurance
Hope the politicians fulfil their promises and provide universal free health care (don't hold your breath on this one, The French have just withdrawn free cover for under 65s not working because it worked out too expensive).
Best of luck. Look in the threads under Healthcare (titled UK and Spain) as a lot of procedures are explained there.