In the 'children twin story', the two twins are surely the main characters. But depending on the nature of the story, there could be other significant characters too. For instance, if it's a story about the twins in a family, their parents would be important characters. Their mother might be the one who takes care of them, tells them stories at night, and teaches them right from wrong. Their father could be the one who plays games with them, takes them on outings, and helps them with their schoolwork. And if the twins have a pet, that pet could also be a character in the story, perhaps getting into adventures with the twins or being a source of comfort for them.
Since it's a 'children twin story', the main characters are likely the twin children themselves.
The main characters in the 'children twin story' are probably the twin children. They could be named characters, like Tom and Tim, or just be referred to as the twins. There might also be other supporting characters such as their parents, siblings if they have any more, or their friends at school or in their neighborhood who play important roles in the story as they interact with the twins.