Is there a subsurface alarm system that will attach to a portable pool?

There is not. It's up to you to test a regular above ground pool alarm to see that it works but don't take a chance with safety. Safety Turtle (company/brand) has a product that your children or pets wear as a watch (or dog collar). When it comes into contact with water a remote alarm sounds. Use any pool alarm as part of layers of protection for your children. (BTW, they have come up with a pool light for above ground pools.)