Hi Danut
The Client could have stipulated that window opening restrictors be provided at least for the duration of the Contract but it is probably a bit late for that.
As Roundtuit indicates, the Client should also have been able to predict that such tenant behaviorus were reasonably foreseeable and COULD have specified a high level of security presence "out of hours", though what would be "reasonably practicable" might not be sufficient to deter all unauthorised access.
I recall stopping outside a housing estate undergoing a refurbishment programme on an October evening and looking at the site security following a complaint. 2m Heras-type fence, bottom ladders removed from each VACATED building, + a couple of secutity guards wandering around.
Next morning I arrived at the office to find details of an accident that night. Teenager from one of the buildings yet to be worked on had broken through the fence, had scaled the lower part of a scaffold [followed by a security guard!] had climbed up to the top platform and had fallen about 10m sustaining head injury [which proved to be not that serious].
So, I started asking QQ to get the comment "Keeping the public away has been a nightmare", the sorts of words that would usually have elicited the response "you could have predicted that when you tendered and costed for the appropriate precautions" but on this occasion I took the comment more seriously.
Turned out that the youths had broken into the site office, broken the new fangled computer [Long, long time ago!] and had been seen off by the secuiity guards.
The following night more youths had broken in this time armed with machetes and the security guards decided that they would [reasonably] take flight.
So, there are limits to what is "reasonably practicable".
So, perhaps yet more education.
The residents may not appreciate that Building Standards require that any balustrade or similar on a modern structure should be at least 1100mm height above the surface from which somebody might fall AND with no gaps large enough to get a sphere of 100mm diameter through.
In contrast, the rules set in the Work at Height Regulations are such that you could easily get something MUCH larger than a 100mm dia ball INCLUDING a toddler falling between mid-rail and toe-board.
Now if this is explained and the odd resident or carer continues to permit children on to the scaffold "out of hours" then may be the landlord needs to take some action.