Untitled

private class Item
{
    InlineElement Element{get;set;}
    int Index {get;set;}
}

var items = new Stack<Item>();
items.Push(new Item { Element = control.FlowDocument.Root });
builder.Append("<html>");

while (items.Count > 0)
{
    Item current = items.Peek();

    if (current.Items.Count > current.Index)
    {
        var child = current.Items[current.Index++];

        builder.Append("<child style=''>");
        builder.Append("child inner text {0}", current.Element.InnerText);
        items.Push(new Item { Element = child });
    }
    else
    {
        builder.Append("child inner text AFTER INNER CHILDREN {0}", current.Element.InnerText);
        builder.Append("</child>");
        items.Pop();
    }
}

build.Append("</html>");