Children are treated differently in law
The attractive nuisance principle recognizes that a child drawn to a hazard may not appreciate it, and it can impose a duty on a landowner even toward a child who was trespassing. Water is the classic example. The duty is to take reasonable steps, not to make the property impossible to enter.
Reasonable steps are mostly physical
Fencing, gates, signage and removing the means of access are what "reasonable" tends to mean in practice. None of it is expensive relative to the exposure, and all of it is evidence that the hazard was taken seriously if a claim ever arrives.
A drowning claim has no useful ceiling
This is one of a small number of residential exposures where the potential claim is limited only by the circumstances. Standard liability limits are not sized for it. An umbrella is the only practical way to carry cover proportionate to the risk.