Also get the window sorted! If you had the radiator permanently off, you could put the cot there. gives you time to sort the glazing. As it's single glazing and the radiator will be off, it'll be freezing come winter so can you get it double glazed?
Dd (nearly 2) and new baby, due soon, are going to have to share room when new one moves out of our room (dd moved out at 5 months). Because the size and shape of the room either new one's cot or dd's bed will have to be next to the radiator and under the window.
There is room, just, if we have the bed next to it, to have a wooden radiator guard over it - this would also serve as handy place to have a clock and teddy and some books. However this would also in theory give a potential step up to the window seal which has a safety lock on but is single glazing so could, in theory, be fallen through.
I have a handy man coming this week to look at it to give me ideas in making it safe (eg a wooden bar across the window sill stopping it being a ledge).
So they question is, is it less bad for the for or the bed to be next to the radiator/under the window? In winter we often have radiators on in the evening but never overnight, and in summer we often have the window open.