Hi
Yes we have the same issue although on a limited scale. Overnight stays are covered within our handbook which allows occasional stays and only after being authorised by the Trustees. So this aspect can be controlled.
We also include in our handbook a provision that residents must be mindful of others and not cause a nuisance. If you have had complaints then I assume a nuisance is being caused and if you have a similar wording in your handbook you can also manage this.
Maybe give a gentle warning? followed by a sterner letter if situation persists.
It should be pointed out that the health and safety of visiting children is the responsibility of the resident, and consider banning them from going anywhere near the water. You are quite right in stating that the Almshouses is not a kindergarten and your beneficiaries are your priority.