Saving ALL nested documents on parent update Mongoose - node.js

Question

My parent model looks like this:
var OrderSchema = new mongoose.Schema({
serviceNotes: {type: mongoose.Schema.Types.ObjectId, ref: 'Service'},
vehicle: {type: mongoose.Schema.Types.ObjectId, ref: 'Vehicle'}
});
The children look like this:
var VehicleSchema = new mongoose.Schema({
name: String
});
var ServiceSchema = new mongoose.Schema({
baseCost: Number
});
I am trying to find an easy solution to updating all of these documents at once. The problem is, when I call an update on an order, it does not update the nested documents. Please see the following:
exports.updateOrder = function (req, res) {
var order = req.body.order,
update = {
$set: order
};
Order.findOneAndUpdate({_id: order._id}, update, {new: true}, function (err, order) {
if (err) handleError(res);
if (!order) return res.status(404).send('Order not found.');
return res.json({order: order});
});
}
An example of req.body in this case may look like this:
{
order: {
_id: 829198218932shdbn,
serviceNotes: {
_id: 8932838nsd2sdnbd,
baseCost: 1
},
vehicle: {
_id: iu283823872378bd,
name: 'Honda'
}
}
}
The order update should also update the serviceNotes with the updated information, and the vehicle with the updated information.
The only way I have been able to update the nested documents is by calling a findOneAndUpdate on the children and updating them one by one. I am looking for a solution to just call update on the parent (order) and have the children update as well.