registerChild(child) {
        if (this.controller.isInitialized) {
            throw new Error("New children may only be registered while the MQTT controller is not initialized");
        }

        this.children.push(child);
    }
Register a new child. All handles will have this implemented, but actual usage will have to conform to the
specifications.
This function must be used as a callback of the controller's reconfigure method so that the state is updated
accordingly.

@protected
@param {MqttHandle} child
@return {void}