Who was watching the children? Adults should never leave a three-year-old and a two-year-old alone.
While it is possible(kids can overcome “child proof” barriers) I would strongly suspect drugs or alcohol abuse in this case.
But then there’s always utter stupidity, too.
They had a security gate up but the children got around it somehow. Seems like no matter how secure you have those things kids find a way to disarm them.