Require young children to be supervised by a responsible adult.

And even then, you will need get a sheriff to basically supervise the actual removal of this tenant and her belongings. 

It sounds like you acted as the general contractor (hired the subs, ordered materials, supervised construction, etc).  

Also be aware of any chemical hazards, such as the remnants of a meth lab or leaking oil/chemicals in the soil.

Also, make sure Dad informs his children of the property rules and supervises the children when they are visiting.

Without any supervision, you are largely at the mercy of the people being nice to each other and getting along.

I know good General Contractors are often a closely guarded commodity, but I was wondering if any Baltimore City investors could point me in the direction of a reasonably priced and dependable GC to supervise the day-to-day efforts of my potential rehab projects.

It might not be the parents, the minor may have other legal guardians and/or the court is supervising her affairs.  

One of which resulted in the discovery of a backpack meth lab which required mitigation in the end for us.  

However, I also have read about a requirement to work under the supervision of a managing broker for a designated period of time.