App.js

const App = () => {
  const [todos, setTodos] = useState([]);

  const addTodo = (e) => {
    e.preventDefault();
    if (e.target[0].value.length > 0) {
      const todo = e.target[0].value;
      const newTodos = [...todos];
      newTodos.unshift(todo);
      setTodos(newTodos);
      e.target[0].value = null;
    }
  }

  const completeTodo = (i) => {

  } 

  const checkboxRow = (todo, i, checked = false) => (
    <Flex
      width='100%'
      justifyContent='space-between'
    >
      <Label
        color='#fff'
      >
        <Checkbox
          checked={checked}
          onChange={() => completeTodo(i)}
        />
        {todo}
      </Label>
    </Flex>
  );

...

      <Box width='350px'>
        {todos.map((todo, i) => checkboxRow(todo, i))}
      </Box>
    </Flex>
  );
}