Being a digital Marketing Agency it might make sense to grant your customers access to the sub-account you created to handle their email marketing campaigns. Giving them the keys to the sub-account means that they'll be able to visit and check all the available statistics and campaign reports.
We're all for cooperation, so if granting access is what you're looking for then you're in the right place!
During the process of adding a new client to your Moosend account, you have the option of creating keys for your client. On the Add a new client page:
1. Tick the Give account access to client? box to give your client access to their sub-account. 
Checking the box instantly opens two additional fields which you'll need to fill-in.
2. Fill-in a Username and a Password which you'll be providing to your customer to grant them access. 
With the Username and Password, your client will be able to manage their sub-account if you're not obliged to do so on their behalf.
To log in they'll have to visit your Moosend page and insert the credentials you provided. Remember that you can easily customize your URL to completely remove any trace of Moosend!