Childproofing: Doorknob help

TinyPinkBug

DH put B upstairs in her room for nap this afternoon, 10 minutes later she walks into the livingroom with a great big smile on her face and says "HI!"  Great, she is finally opening door handles.  And sure enough, later on she kept opening the front door!  I have regular doorknob covers on several doors, but there are some doors that I need Ty to be able to open on his own, but not Brianna.  Has anyone found any door knob covers that might prevent a 1 yr old from opening the door, but not a 3 yr old?  My internet search turned up nothing.