Tell us then what is the difference between programming a mini PLC and programming a Raspberry Pi running CodeSys from your perspective?
From my perspective here are differences, but they'll be in the realm of instrumentation and electricity and nothing at all with actually programming a PLC.
Also, most of the cheap PLC options have ladder as a standard and if you're lucky you get function block diagram. The Raspberry Pi lets you program in 5 different PLC languages, all according to IEC61131.
If your interviews are as deep as "tell me what PLC's you programmed", then you're asking for trouble... but that doesn't even matter as Colt Hero isn't looking to get his son a job based on this, but instead is looking to broaden his horizons to industrial controls.
However, all things considered, if Colt Hero's son does go on an interview after only playing with this PLC (whatever the choice), he will say that he did a home project, period.
Going with what is a full fledged development will allow him to test and give an example of using ST or SFC, whilst going with a normal PLC will not be the case.
Perhaps it's just me, but if someone says they can understand Ladder only, they are in the bottom of the pile when it comes to PLC's.