Small Beach is the main beach in Ulcinj city. It's pretty wide sandy beach located in a little bay so there are no big waves mostly and it's appealing for families with kids.
There are areas with paid sunbeds and umbrellas as well as a free part of the beach where you can rest on your own, play with kids on the sand. The sea is great there - not too deep nor too shallow but it's perfect for kids and their parents. There are a lot of supermarkets around, cafes, restaurants and other infrastructure. It's very close to the border with Albania so you can travel there as well.