Lock all exterior windows, doors and sliding doors at all times. Keep the keys well out of reach of children. Install self-closing mechanisms on doors. Don't let these give you a false sense of security, however. Lock doggie doors as well.
Small children can easily fit in the small openings of doggie doors.
Lock back gates and front doors. Don't place chairs, tables and other objects near pool fences. Children can use these to climb over. Better yet, place outdoor furniture inside the locked pool fence where it will be inaccessible to children. Empty wading pools when not in use.