Door proofing that a 4 year old can work, but a 2 year old and 1 year old can't?

So, my two year old son knows how to work a doorknob. He has autism and he will go in the bathroom and turn on the sinks.

But I also have a potty trained 4 y/o who needs to be able to get into the bathroom. My 4 y/o can work a baby gate latch, but the bathroom door is too narrow for a baby gate.

We had these about six months ago but his hands were too small to use them:

I'm going to buy some more and try them again, but I think his hands are still too small.

Right now, I'm having to lock the door and just let my 4 y/o in and out every time he needs to pee, but that is getting old fast! The door isn't a key lock, it is one you unlock with a butterknife or screwdriver. No where I can leave those that the 4 y/o can get to but the 2 y/o can't.