The parents would find themselves in an awkward sitution should they initiate lawsuit over this. The fact that they were there but didn't have their kids supervised pretty much guarantees they'd be considered at least partly responsible for their child's death. There is no way they could prevent that fact from being brought out in the trial, and the attorneys for any other parties sued will hammer that point every chance they get. There is a chance you might be named as a defendant in the lawsuit; typically the first complaint filed will name anyone who might possibly have some liability as a defendant in the lawsuit. However, for you to be liable, the parents or the estate of the child will have to show that you were somehow negligent and that it was that negligence that was the cause of the drowning. If the plaintiff can't point to anything that would be negligence on your part then even if you are named in the complaint, you'd likely get dismissed as a defendant early on. If they can put some kind of theory how you'd be negligent, even a weak one, then you'd end up having to defend the claim. How far the claim would go in that circumstance is impossible to guess without knowing what theory of negligence they'd put forward and what evidence they have that would support it.
I suggest you notify your homeowner's insurance company of what happened. It likely has a duty to defend you in this matter, and your policy likely requires that you notify the insurer promptly of anything that may trigger that duty. If you delay, the insurer might use that as an excuse to deny all coverage. If contacted by anyone about this tragic accident, don't talk at all about what happened that night and refer them to your attorney or tell them you'll get a lawyer and provide your lawyer's information later. If you don't already have a lawyer from the insurer, promptly notify the insurer of the contact so it can deal with it.
Out of the three persons who might be liable for this, I see you as at the bottom of the list. That doesn't mean that a jury might not find you partially liable somehow. But it does mean that the owner of the Air BnB and the parents are likely to get hit with most, if not all, of the damages awarded out of any lawsuit.
