var tree = [

    { name: 'node1' },

    { name: 'node2', children: [{ name: 'node-2-1' }, { name: 'node2-2' }] },

    { name: 'node3', children: [{ name: 'node-3-1', children: [{ name: 'node3-1-1' }] }, { name: 'node3-2' }] }

];

var key = 'test'

function parseJson(arr) {

    arr = arr.slice()

    function toParse(arr) {

        arr.forEach(function (item) {

            if (item.children && Array.isArray(item.children)) {

                item[key] = item.children

                toParse(item[key])

            }

            delete item.children

        })

        return arr

    }

    return toParse(arr)

}

console.log(parseJson(tree))