"...if had been properly secured, it wouldn't have toppled off."
Have you actually been to a museum or art gallery? Vases, for example, are not 'secured' (how would you secure a vase anyway?), and very rarely they get smashed:
www.theguardian.com/uk/2006/feb/06/arts.artsnews1
But at least in that case, the chap fell, rather than, for example, a boisterous five year old who had decided to climb up on the window sill. In your own home, with your own objects, you can secure or tie down things as you will. But in a space like the one shown (not a totally unsupervised public space where you might expect kids -- the naughty ones anyway and even some adults to clamber on statues), you expect unsupervised kids to know the difference between:
a) furniture
b) play objects
c) decorative objects
or they are not fit to be left unsupervised.
So, just as I'd not chide a kitten for climbing up the curtains, but I'd expect a child to be old enough to not do so, so I'd expect a child to know not to climb up on this object. If they don't know that, they either need to be supervised, or taught the differences as above.
And of course, we all have plenty of experience of seeing kids in public spaces, where in full view of their parents (or even with the parents themselves joining in), keep off, do not cross this rope etc signs have been ignored. Including one sad case in Norway where parents disobeyed a clear 'no closer to the glacier' barriers etc
www.thelocal.no/20140811/tourists-die-in-norway-glacier-tragedy
You cannot make everywhere safe, you cannot secure all artworks (apart from keeping everything behind bullet-proof glass and everyone six feet away -- or better still, don't display them at all!). I don't know how anyone expects someone to 'secure' a vase, or a sculpture. Apart from having a 'do not climb on the exhibits' sign, or expected parents to be responsible, what more do you expect? If it is an area that children do not enter unaccompanied (unlike, say, a park or public square), then I would expect a reasonable standard of supervision or behaviour. And that you deal with the consequences if your lack of supervision causes damage.