i) A whole year can pass with no questions about Murcia and then there are two in as many days! Has it been on a TV programme? There are guide books covering Murcia (region & city), I can recommend, but since your stay will be brief I suspect they are not worth the investment (perhaps your library has copies of, for example, Rough Guide or Lonely Planet Spain books?)
ii) I am going to assume your real intent is to get from Valencia to Granada and you want a brief stop. There is, as it happens, a lot worth seeing across Murcia region (especially natural sights), but you'd need several days for that.
iii) So, as a stop I would suggest a single night in Murcia city. The following is about the city, not the wider province & region of the same name
iv) Murcia is certainly convenient (roughly half-way point), on your trip. But, I think Granada deserves at least two full days (I.e three nights), so I would take one off Murcia and add to Granada.
v) For sights in Murcia city during your overnight I'd put top the Real Casino (not a casino in the modern sense, but a private club with beautifully decorated rooms in many styles), the cathedral (see below) and the Santa Clara Convent. There are also a few bits of Roman wall and the covered market is worth a look. But the best thing about Murcia is perhaps being outside and enjoying the squares and streets. I cannot, for example, remember the cathedral's interior (they mostly merge into one after a while), but the exterior is stunning as is the square it faces. Flores square is another delightful one and so is walking along the river. The main advantage of being outside is you can visit some of the great cafes and restaurants. Murcia is a "foody" paradise.
vi) However, be aware that in August, it will be scorching hot. And I realise none of the sights above might appeal to your sone. For something completely different, on the city outskirts is Terra Natura, a sort of zoo/wildlife park with Iberian and non-native species and which includes a waterpark.
vii) If you want to break your journey from Valencia to Murcia, you might make a quick "pit-stop" in Elche to see the palms. Similarly, from Murcia to Granada, I would take the route via Lorca, Baza and Gaudix. I'm not suggesting you stop at all (or indeed any), of these, but the route has some nice views and opportunities to briefly stop to stretch your legs, take photos or have a snack. Your son might like the Guadix cave homes.
viii) I'm completely ignoring Cartagena here, but that is an alternative to Murcia city, without adding much time overall. Its sights might be of more interest to an 11 year old (seaside, submarine, sunken treasure, castle on a hill, Romans, air-raid shelter). Happy to say more, but I suspect I have blathered on enough already.
