Designing something in such a way as to make it's operation or use difficult by those with low mental faculties (e.g. children). The basic idea is to make it secure by requiring logic, reasoning and/or literacy; things that most young children lack. Notice I said most children.
This comes in many forms, however the two most well know would have to be pill and chemical bottles ("push down and turn") and cigarette lighters ("slide lever to release butane valve").
Child Proof can also be used as a verb, as in "we are going to child proof the house" which involves covering electrical outlets and putting up baby gates.