Hi Mike,
So, I assume you are trying to get something as simple as a GetCampaigns example to work. If that's the case, then only an advertiser account can have campaigns, MCC accounts cannot. But based on your description alone, I cannot tell for sure if the child accounts under your MCC are advertiser accounts, or MCC accounts themselves.
My suggestion is this:
1. Set ClientCustomerID to the customer id of your MCC account (it is on the top right corner of the website once you login into the MCC account.
The script will print your account's hierarchical structure. The accounts at the leaf level are usually advertiser accounts, but you can also navigate to these accounts by clicking the account listed under My Client Center tab of your MCC account.
3. Keep looking until you find an account with campaigns in it. Copy its customerId to ClientCustomerId. Now run GetCampaigns.cs, and you should get the campaigns within that account.
Cheers,
Anash