North (alghero) or south (villasimius) with two little children

We have never been to sardinia, but we would like to go there in July. We will travel with 2 and 4 years old children.

Both airport is oke for us and willing to rent a car as well.

Looking for slowly deepening, really warm beaches within walking distance without crowd. We will take a trip but not every day, we would like to go for a boat ride as well.