If parent has no children, then set loaded attribute as true in JSON responce or root(in your case).
your root config should look like this
root: {
text: 'Root Node',
expanded: false,
children: [{
text: 'Child 1',
leaf: false,
loaded:true
}, {
text: 'Child 2',
leaf: true
}, {
text: 'Child 3',
children: [{
text: 'Grand Child 1',
children: [{
text: 'Grand... you get the point',
leaf: true
}]
}]
}]
}