When our daughter developed the ability use doorknobs, we wanted to make sure she didn't enter certain rooms of the house unsupervised. We invested in several doorknob covers to keep her safe.
These covers come in two pieces. They snap over the doorknob and have to be squeezed fairly tightly in order for the cover to come in contact with the doorknob. Once you squeeze the cover, you can twist the doorknob and open the door.
These are very effective. They are difficult to manipulate, even for adults. In fact, we have to remove them when my elderly mother-in-law visits us as she cannot squeeze the pieces together hard enough to engage them.
One drawback is that they sometimes fall apart and end up on the floor. This has only happened a couple times after they were hit hard with something we were carrying. I have no fears that our children can make them come apart on their own.