How do I manage a client's sub-account balance, auto recharge and credit limit?

Open the sub-account (it is also a contact in your CRM) and go to its Billing tab. There you see everything related to its billing, usage and auto recharge. You can view and edit the balance, turn auto recharge on or off, and set a credit limit.

By default a client cannot go negative for most services. For things like sending SMS and making phone calls, the service turns off when the balance reaches zero, unless you set a credit limit that lets the client go negative by a set amount. There is no credit limit to start with. Incoming items that cannot be blocked, such as an incoming text message, can still take the balance negative.

For your own sub-account you can set a high balance, since you would otherwise be paying yourself. There is no spending cap setting.

When you create a plan for sub-accounts, put at least a dollar or two as the one-time credit so new customers do not start out locked.