To get around Sardinia, I recommend renting a car so you can explore the island and visit beaches such as Poetto. I went in March and I did not think when I dove headfirst into the water and it was a little cold but having the beach to yourself alone or with your partner is a luxury :) There was very clean water, fine sand, but it was full of shells, so don´t forget to look down towards the ground before you take a step!
In my opinion, Calamosca is one of the best beaches in the Cagliari area. It's a small beach where you can unwind and escape the crowds of the bigger beaches. If you're fond of kite surfing, then this is your spot! If not, it's also a lovely place to commune with nature throughout the year. The sand is fine and white but there are occasional rocks, so take care, particularly if you're with kids. If I had to make a criticism, it would be that it's quite out of the way - the bus doesn't stop too close to the beach, and it's a bit of a walk from the main road.
Whoever finds himself in the dilemma of going north or south, I advise you to do a combination of the both. There are so many good reviews of the north, but the South of the island has some really stunning, white sandy beaches and calm water, suitable for children because of it's tranquility and walking tourism, as well as other activities like hiking and mountain riding etc. Everything is surrounded by sand dune beaches that are protected for their ecological value, which makes snorkeling on the beaches of Pula, Chia, and its surroundings impossible. Frankly it's worth spending a few days on this part of the island.