We have a baby latch on teh fridge because the 4 year old climbed in and tried to shut the door so he could jump out and scare someone like on the opening credits to AFV. I totally freaked. The 7 year old can easily open them but the 4 year old can't.
The doorknob protectors work well. If they get dirty then the rubber will slip and you'll have to replace them. We had them on the outside of our bedroom so we could have some warning before the door pops open.
We have them on the bathroom doors, well the kids are potty trained now and they are not still there but they were until 6 months ago. K, the 7 year old, can open them by sticking her finger inside but J cannot figure them out yet. I give him 2-3 months. When my daughter was a newborn I worked at CR anthony's in OKC and a lady came up to me one day, she said I understand you have a new baby at home...then she started tearing up and said her baby had drowned in the toilet at about 18 months. She begged me to promise her I would always keep the bathroom secure and that has stuck with me all my life. I have real issues with the bathroom bing inexcessible until they are potty trained and able to pull their body out of a toilet/bucket, etc...baby and toddlers do not have the neck muscles until older to lift their head out of the toilet bowl if they fall in. So, keep the bathroom and other areas with water in containers secure.
We did, and still plan on doing, attaching the shelving units to the wall studs with L brackets on top, at least one and maybe 2 brackets if the stuff on the shelves is very heavy. Then also each individual shelf has tiny L brakcets insted of any peg type holders. Kids climb, shelves get unbalanced, shelves collapse and those little plastic pegs break. L brackets make much better sense.
Plug covers, kids figure out how to remove the simple plug in ones so we finally bought the better ones that are difficult for us to remove.