My kid is potty training and still needs help sometimes, but he keeps locking the doors.  I got tired of looking for something to unlock the door with, so I designed this.  It can stay installed on the door and easily removed while we have guests.

You can print this in two pieces without supports.  If you print as a single piece you'll want to add supports just above the spring as shown.  

You'll want a fine layer height so that the spring isn't too chunky and compresses well.  I printed with a 0.12mm layer height and 4 walls with a 0.4mm nozzle.  It's quite tall and narrow, so maybe consider a brim as well.

You may need to actuate it a few times after printing to loosen it up, especially for the single piece print.  If you need to change the hole size, rod length, travel distance, tolerances, etc the attached f360 file is parametric.