How would one trigger a deployment plan for the parent build plan, from a child dependency?

[Jason Eala]

Suppose we have build Plan A and build Plan B. Plan A is the parent of Plan B. When Plan A finishes successfully, Plan B will be executed. Upon a successful run of Plan B, we want to trigger Plan A's deployment to the next environment. How would one go about doing this second part of triggering Plan A's deployment plan?