Malaga area. Fuengirola, Benalmadena, somewhere along the coast between the airport and Fuengirola because that's as far as the train from the airport goes. Loads of flights from most UK airports.
I can't recommend any accommodation in particular, but I'm sure you can use booking.com or similar. There are big AI hotels in places like Benalmadena and Torremolinos, but there's also loads of other options. Benalmadena has a cable car, big park, there's a couple of waterparks in the area too which might still be open.
Train line from airport to Fuengirola and also into Malaga city. You can get a bus to Mijas, which is an inland white village, with hiking routes on the outskirts (but be aware that most are too advanced for small DC - we go walking and mountain biking in the area a lot and the mantra is always 'steep, rocky and loose'.
But Fuengirola has a castle and you can walk inland along the river, through parks and picnic areas. There's a skatepark, where they can watch even if they don't skate themselves. Also play parks, can walk along the seafront for miles in each direction.