Plavi Horizonti Beach is one of the best beaches in Montenegro, I would say its the best one! It's a sandy beach which is pretty rare in the country. It is located on Lustica Peninsula on Adriatic sea. There is a very big paid parking next to the beach (3 euro per day) and a free one a bit farther.
It's a perfect place for the rest with kids - sand allows them to play different games, build sandcastles and much more. It's a pretty shallow sea near the beach and that's why it's always a much warmer sea than in other regions. It also makes the sea pretty safe especially for kids. If you like to swim in deeper water, you will need to walk into the water like 15meters or more to reach a deeper part of the sea and enjoy swimming in a very warm sea.
The beach is located in a natural bay and it's protected from the wind and storms. You can find rent sunbeds and umbrellas on the beach, there are changing booths, restrooms, small children's playground, a nice restaurant, and a beach bar where you can have a good meal, or buy some drinks or whatever you might want. Everything you might need on the beach is there. Changing booths are actually very good ones, very protected from others.
There is a nice promenade that leads to the forest. You can find there a walking trail along the sea, to a hidden stony beach. It has a wonderful view and a nice private swimming time.