With written permission from Mass_Maker before posting:

This is a remix of his child door latch design.  I printed the original and found that as the latch was pulled, it would deflect and open! (oh no).  Unfortunately my child was just strong enough to exploit this……. so I modified the latch design to keep the intended function, but it now should strain tighten (lock harder) as it is pulled instead of opening.   This is still just 3d printed plastic held on with tape……. so no results are guaranteed at all, and something will eventually break.   I take no responsibility and guarantee nothing with respect to safety, reliability, or function.

This uses a medium size command strip for the catch, and a large command strip for the door side.  

Note that the first rotation of the on/off toggle will be VERY tight.   That's mostly intended so the toggle doesn't just sag out of the way.  exact firmness with vary with your print settings.

I also suggest mounting this as shown in the photo (side mounted).  This will result in the entire latch being loaded instead of the outer portion of the latch hook carring most of the load/strain.

Disclaimer:

*while this child safety lock design is free and intended for personal use, I cannot assume responsibility for its manufacturing, distribution, or any potential misuse or failure to assure child safety.