I don't understand how you can update jms in a child site.
The update must always be performed on the master and I remember you that normally all the PHP code is normally the same.
The slave site (child) normally share the files and folders of the master.
I don't understand why you want to modify the config_multiisites.cfg file that is an internat JMS Multisites configuration file where are stored the paramters that you setup in "Manage" menu.
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