The config file does not (directly) affect the connection string in the child package. The child package inherits the value of a variable from the master package. You then use that variable to build an expression to set the value of the connection string in your child package. Have a read of this for more information. Once you've created your packages like that, the only way you can set the value of that variable is to call the child package from the parent, therefore if you're running it in BIDS you need to run the parent package and wait for it to call the child.

John