You can do this using of the community SmartApps. @bravenel’s Rule Machine is very good for this. You can generate two rules. One to let it notify you when they arrive and you are not there, and another one to notify you when they leave if you are not there. You just need to create a routine that does nothing else than to notify you and have the rule activate that routine.
Here is a guide to how to use community SmartApps, by our own @JDRoberts.