I don't know about a lock, but you could always place a piece of wood between the top of the lower sash and the top of the window, like a wedge if you can follow my description, to prevent the lower sash from being raised without the wood being removed. Anyone on the inside could then remove the wood to raise the window, but it would be nearly impossible for someone on the outside to budge it.

Only problem with that solution, is that if the unit is a double hung, the upper sash will be able to be pulled down. Your options are limited, because you need to keep the egress in tact.
You don't mention how old the child is or what type of windows you have, but here's what I've done:
On some old wood double hungs, we positioned the sash to the spot we wanted them to lock, about 4 inches of opening. We then drilled 1/4" holes through the lower sash into the upper sash. Our sash were 1-3/8" thick, so we drilled 2-1/4 inches. Then we took 2 old hinge pins and slid them into the holes, locking the sash together. To restore full operation, simply pull the pins.
This will not work with modern clad windows, as you'd be drilling through the cladding. That will void warranties.
Many hollow vinyl windows have a pop out tab on the upper sash that will limit the opening to a few inches.
Being a parent myself, I'd probably keep the window locked and install central AC or a wall mounted unit if I had any concern for safety. Besides, if someone wants in, they will come in. With the window locked, an alarm could be installed. They have some small window alarms that don't require a whole house system. It's worth the piece of mind.

Mike & Squeak -

Thanks for your comments. You're right - if someone wants in they will get in. We're in a very safe neighborhood in a safe town, but crime happens everywhere. I agree that the first solution could be dismantled too easily by lowering the sash.

I have AC but I like fresh air, as well as two dogs who will and do bark at noises but they don't sleep in my 10-year-old's room. I've heard of Mike's solution but wondered if there was a manufactured lock or something out there.

We'll probably go with the drill hole through both sashes - It will make a good project to go along with the spring window cleaning. And it's another reason to drag out my new drill!

Thanks!

I didn't think about the upper sash. I keep forgetting that other people have working windows which actually open (unlike ours). :)

Hi - I'm just getting back into this post! I've had to detour about roofing, etc. It never ends!

Guy - Thanks, I'll look into the sash locks. I'll contact you if I don't have any luck.

Nellie

There are devices called "vent locks" which allow the window to open only to where you set it- generally too small for the average human head as per the instructions. The most secure ventilation for a wood double hung would be to immobilize the bottom sash (with an l-bracket or piece of wood as above), then vent lock the upper to limits its opening- freash air bue impoosible to get iin! Most housebreakers do not want to break glass- too much noise.

I know this post is old, but it speaks exactly to my question. I have new double hung windows and we bought a cat window that goes the width of the window. It is installed at the bottom of the window and the lower sash sits directly on top of the cat window. We bought locks to keep the bottom sash from being raised since we lost the original locking feature when we installed the cat window. However, we discovered last night that the upper sash is no longer locked and don't know how to remedy this since there isn't a track at the top. Any suggestions?