Having had 4 break-ins over the years, and now having a lot of first floor windows unoverlooke at the back, giving onto a single storey extension, we have window locks - in use - on every window. However I have the key to unlock them in every room adjacent to the window, but unobservable from outside.
The purpose of window locks etc is to stop people getting into the house, not stop them getting out and once they are in, providing the main doors cannot be opened easily to remove loot, they then have to go out the same way they came in.
I have always had keys accessible by the relevant window, even when my children were small, but I explained to them clearly why the keys were there and the sanctions that would be imposed if they were touched. other than in need. They were never played with.