the element type 'List<widget>' can't be assigned to the list type 'Widget'

Here you are wrapping a list as list

children: <Widget>[getList()],

This should rather be

children: getList(),

Since the answer was not satisfactory for me, so I found another solution.

Flutter gives you the possibility to write conditions on a single line also in the graphic, so you can insert a cycle on a single line and then insert the other different widgets. This is a practical example in response to the main question:

 Column(
    children: <Widget>
      [
        for (int i = 0;
        i < workouts.series.length;
        i++)
          "//CREATE N YOUR WIDGETS//",
          new Text(
          "${workouts.series.length} x ${exerciseWorkout.repsToDo} ",
          style: TextStyle(
          color: Colors.black87,
          fontSize: 16,
          fontWeight: FontWeight.bold),
          )
      ],
     )

Just use spread operator:

return new GridView.count(
    crossAxisCount: 2,
    padding: const EdgeInsets.all(10.0),
    crossAxisSpacing: 10.0,
    mainAxisSpacing: 10.0,
    children: <Widget>[...getList()],
);