I purchased the Clear Grip Door Knob Covers by Safety 1st when my son began trying to open doors. He has not mastered door knob opening yet, but I still use these for when the moment comes and he can open them. I keep some door knobs uncovered, but for the doors leading into bathrooms, I use these. My son likes to unroll toilet paper (what small child doesn't) and loves to open the toilet lid and put stuff in the toilet! Now, he can't because he can't get in the bathrooms.
The Clear Grip Door Knob covers come in both a 2 pack and 3 pack. They are very easy to intall. Each cover comes in 2 pieces that easily snap together over the door knob. To open the door once installed, press the rubber buttons on both sides and turn the door knob as usual. The door easily opens. For a toddler, they can't press the buttons so the covers just move over the door knob without turning it.
I would recommend these to the parents of any toddler. This will help keep your child safe.