JSON model for TreeGrid Treecolumn component in c#

Question

How can I build a dynamic JSON model for ExtJs TreeGrid Treecolumn component in c#

format response:

 root: {
    text: 'Root',
    expanded: true,
    children: [
        {
            text: 'Child 1',
            leaf: true
        },
        {
            text: 'Child 2',
            leaf: true
        },
        {
            text: 'Child 3',
            expanded: true,
            children: [
                {
                    text: 'Grandchild',
                    leaf: true
                }
            ]
        }
    ]
}