Torrevieja Aqua Park may be a bit hectic for the younger ones. It is great for teenagers, though.
There is another one at Quesada which is straight up the AP7 from Murcia and is suitable for all ages. Also one at La Siesta (Called the Flamingo) which is great for younger ones and suits up to about age 11, plus it is cheap.
Plenty of horse riding in the area which could appeal to the older ones. My grandaughter has been going horse riding each year since she was 8 (now 10) but had previous experience in UK.
I know of a go-kart track on the N332 in Playa Flamenca. The new underwater archaeological centre in Cartagena is well worth a visit and even impressed my 13 year old grandson.
Also lots of fishing available (lake) at Torremendo which is also a great area for walking and hiring out a boat.
4 to 6 weeks is a long time to keep them occupied. Safari Park at Elche (nearer to Santa Pola than Elche) OK for a day out. About 2 to 3 hours drive to Terra Mitica (near Benidorm).
HTH