You need to know where in your hierarchy exists the row you're after, including a reference to each of its parent rows - all the way up to the root row in band 0.  Each row has a collection of its own child rows.

For instance, say you want to loop through all rows within bands 0 and 1 of a grid.  The following JavaScript would be your basic setup:

// Assume that "grid" is a reference to your WebGrid instance
var rowCount0 = grid.Rows.length;
for (var i = 0; i < rowCount0; i++)
{
    var row0 = grid.Rows.getRow(i);

    // Do something with the band 0 rows here

    var rowCount1 = row0.ChildRowsCount;
    for (var j = 0; j < rowcount1; j++)
    {
        var row1 = row0.getChildRow(j):

        // Do something with the band 1 rows here
    }
}

The getChildRow() method is one way of getting at the child rows of a particular parent row.  It iis likely one of the "cleaner" approaches, from the perspective that it's relatively consice and easy-to-read.