These are the nicest family (and parent) friendly beaches we found on our travels:
El Cristo Beach in Estepona
Directly on the beach there is a small, shaded play park which my son loved playing on. There's a little beach bar called Havana Beach Bar Chiringuito which sells beers (yes!), wine (yes), ice-creams and restaurant food. So once you get there, you don't even have to leave.
Plus, the beach and water is super clean. We got down there are 9am one day (yes, we are keen) and there was a tractor down there ploughing and flattening the beach. Which my son especially loved! The beach was never busy, but perhaps that's because we were down there so early everyday!
Playa de Levante in Puerto Benus
You may not think that Puerto Benus would be an ideal place for children. But, actually, Playa de Levante or Playa Puerto Benus is really family friendly.
Another sheltered cove, this water is calm and shallow (so slightly warmer than some other beaches) and the sand is fine and not too rocky.
There are lots of bars and Chiringuitos along the beach, but we struggled to find one that wasn't overpriced and over styled. In fact we had one of our worst and most expensive meals at the Levante Beach Restaurant. Much better to bring a picnic lunch down and chill out under one of the umbrellas.
El Faro Beach in Marbella
OK, so this isn't exactly the most picturesque beach in the area and, because it's just off the centre of Marbella, the clientele can be a bit, well, suspect. However, if you're looking for a beach that's perfect for Mummy and for toddler, then El Faro Beach would take some beating.
The beach has a large play park planted in the centre of it which is completely in sight of all the bars and restaurants. Meaning Mummy can have a little drinky while Daddy plays with toddler on the slides (and sometimes steps in to help).
You're spoilt for choice for family friendly beaches in Marbella and Estepona, but these are the ones we especially enjoyed. Spain does this so well by combining adult playtime with family playtime. Wish we had a bit more of this sort of thing here in the UK!