Answers for "center child div"

text-align:center apply to parent div or child div

.parent {
    width: 100%;
    border: 1px solid blue;
    text-align: center;
}

.child {
    display: inline-block;  
    border: 1px solid red;
    margin: 2px;
}

center child container

Container(
        width: double.infinity,
        height: 500,
        color: Colors.amber,
        alignment: Alignment.center,
        child: Container(
          width: 200,
          height: 200,
          color: Colors.red,
        ),
),