I think the T could fill in the pond too, but return it to a pond when they moved out.
Right and I specifically said it would probably be costly but if that's what they wanted to do, that's up to them. I never mentioned there being anything legally wrong with it - just that it was surprising that they wouldn't have spoken to the landlord beforehand as I think if I were going to do something like that, i would want the landlord to know...but that's just me and obviously other people wouldn't feel the same.
My OH filled in the pond in the house we lived in, when we moved out and then rented it, as LL is responsible for the safety of their tenant.
I definitely wouldn't have questioned that the LL is responsible for the safety of the T...which is why I would expect, if the T thought there was a risk for her grandchildren, she would contact the LL and speak about how best they can mitigate that risk (whether it be perhaps a fence or cover or what). Again, I didn't say they had to legally do this, but it's surprising that they appeared to make zero effort to speak to their landlord about the concerns as well as deciding to do home renovations that had nothing to do with tenant safety
In isolation, I wouldn't have been as shocked. Hearing about the extent of what they've done, as a whole package, that is what makes it surprising.