Not sure if this is much of an answer but I did see those adjustable "fence" things in Fair Price at Bukit Timah yesterday.
Do you know the ones I mean?
They are designed to go in doorways or at the top of stairs and are adjustable width-wise. Perhaps they will fit into the window frame? Maybe either at the bottom of the window so that your darling cannot reach the latch, or right in front of the latch? I am pretty sure that the bolts (screws?) that hold them in place would be strong enough to suspend it.
I have also seen many types of Child safety products for doors, windows etc but exactly where I saw them escapes me just now. I will come backand re-post when I remember.
Good luck!
Mabel