The rectangular ones I'm familiar with are the Decora style, made by Leviton. I don't know of a child-proof cover for them, but the ones I've gotten recently from Lowes and HD have the Tamper Resistant feature built into the receptacle itself. A shutter prevents the insertion of an object into only one of the slots.

Here is a link that might be useful: Decora Plus Duplex Receptacle

cwyn,
Is this what you are describing?
{{!gwi}}

If so, their purpose is to keep the weather out not little kids. You'd probably make the outlet more interesting (and therefore less safe) to little minds if you put them in your apartment.

Never mind. You're probably talking about these things in the link. (Can you tell I don't have kids?)

I found it by just searching for "decora child proof outlet covers". I have no idea if this is a reputable source.

Here is a link that might be useful: Decora outlet covers

Thanks for the response, randy427. Our outlets do look like the link you showed, but I'm not sure if they are tamper resistant. Since they were just replaced, they might be required by law to be tamper resistant, but I'm not sure what the rules are for New York City renovations. Is there a way for me to tell if the ones we have are tamper resistant?

greg_2010, thanks for the link - this is indeed exactly what I was looking for. So I guess the name of the shape of the outlet cover is Decora - this makes it much easier to google what I'm looking for. :)

Thanks so much to both of you for your replies!

There are both sliding and rotating tamper resistant outlets.

Or you can just keep an eye on the kids like most folks did for a very long time.

These days you're pretty much required to put in TR receptacles in most new work. I'd definitely look into those. They're more reliable than just about any "add on" cover you can put in place.

Thanks for the responses brickeyee and ronnatalie. Does anyone know how I tell if the ones that we have are already tamper resistant?

I think that if they had that type of outlet when I was young, it would have made them much more interesting to me, a better toy so to speak.

They will be marked as such if they are TR. Many of them will have TR molded right into the plastic of the receptacle you can see just by looking at them (frequently between the holes. Others you may have to remove the cover plate and see if it's stamped on the mounting tabs. Most likely you don't have them if your house is older, they've only recently become mandatory and that means that builders DIDN'T bother using them before.