A 1/2 inch piece of wood with knotches in it will allow the window to be propped open at various heights. You can push tight against the side jam which should be safe.

I've seen a latch type device that attaches to the window frame and when the lever is pushed down, it has a "foot" that puts pressure against the window sash holding it in any position. YOu may find at a hardware store or one of the box stores. Safer than props, pretty hard to have your head under the window and reach up to unlatch at the same time.

All of our windows have varying degrees of slip (or make that slam) down-itis. Scares me for my pets safety.

We use a variety of things. The most elegant are notched sticks about 1 1/2 wide with a succession of half-width notches up and down the stick. This is set right against the jamb.

However with inquisitive little fingers in a rental I would get a selection of 1 inch wide boards, cut to appropriate lengths and slip one tight up against the jamb in the groove where the window would be if it was down and where it would be complicated for a kid to pull it out. Top remove this stick, you have to lift the window's weight up and off the stick, in order to close the window.

You could also fiddle around with the little sliding pegs to make sure they are snapping as far out into the little holes as possible. However, often the little holes are very worn and allow the pegs to slip out. If the peg is missing, you can try large sized nails, of an appropriate diameter, inserted into the window frame and through into the jamb. In a pinch I have used double pointed knitting needles, broken chopsticks and the pronger from a french twist hair clip. (Also, like you, books, soda cans, wooden hangers, fireplace tools, sticks of kindling, salvage pieces of lathe .... I can get quite creative.)

Nothing will keep me from fresh air at night.

Molly

...I'd just like to add, even though you said landlord involvement isn't likely, that your landlord really should WANT to be involved as we're talking about a major safety issue here. If he's going to rent to people with children, it's foolish of him to not replace or repair the windows. I'd think his rear would be on the line if god forbid something awful should happen. If he doesn't care about the safety of his tenants, he should at least care about his pocketbook and his potential for being sued. JMO.

you should at least ask him for a credit against your rent for whatever you wind up doing to the windows to make them safe.

Thanks for all these great ideas - the helmet idea is good but I might want to fashion a plastic bubble for the kid, as that would cover even more bases. So it looks like I should make my own device, right? The landlord thing is very sticky since we're renting from a family member (this is always a bad idea and I protested before we moved in but it fell on deaf ears. It's been an adventure of peeling lead paint, backed up septic system, ceiling cave-ins, etc.)

I use the pieces of wood against the jamb. I use a piece of wood on both sides of the window. So even if one piece of wood is dislodged, the other side will still have a piece of wood. I cut the wood to a length that makes a tight fit. I think I'd worry more about the child falling against the screen and out the window than the window sash hurting him.
Also, have you done a google search for products related to childproofing, child safety, etc. I think it is an industry unto itself these days, with all kinds of gadgets. How did my mother safely raise 4 kids without those little plastic covers that fit over the electrical outlets?!

What about plain old screens?

We used the aluminum strips for several years before we replaced the windows. They actually work pretty well and it's an inexpensive fix. They're bent in the center and they fit on the inside of the frame and the window depresses it as it goes up and down. The depressed strip puts pressure on the window and frame and holds it in place. Makes them a little harder to open and close but they Do stay put when they're open. You can use a strip on each side for really bad windows.

Getting your hand slammed in a window HURTS and causes major bouts of swearing while doing the ouchie-pee-pee dance.

Cricket

Hey, I may be the overly protective parent here, but two year old and open windows with screens sounds like the makings of some bad times if the windows are elevated at all. My two year old loves pushing on screens not realizing that he can fall right through. Just a thought.

Interesting that no one mentioned replacing the ropes in said windows, but I agree if that is not practical, the wood sized to fit next to the jamb, and especially one on each side seems like a good solution.