Childproofing dc's bedroom windows for the summer.

[Bringiton2016]

Hi, still saving up for new windows but they will have to have another summer in their oven rooms in the meantime. The old upvc windows only open all the way. Is there something I can put on them so they only open half way so they can still get lots of fresh air but they cant fall out?

something like this

or this

Wow brilliant! Cheap and simple too! Thank you. I'll check they're compatible.