I dont know about up in the mountains but I have taken my 4 year old to the montana roja site. It is great as it has all the amenities that you may need as well as there own tents
There is access onto the beach and if they want to do some exploring there is the rock pools further along the beach.
I know he isnt after a campsite but I think you need to be careful where you pitch your tent, especially in the mountains