It's clearly a graphical problem, but I don't know how it's work.
[...]
My 'child' Spitter uses different armor than its 'parent' because it looks different (shell-less). I don't think using the same armor is the cause of the issue, but you can quickly make another armor by duplicating the original one, to test if that's the case.
I didn't have any problem with the Spitter spawning; I also remember making a Reaper spawn a Chryssalid for testing purposes, without issue (was quite some time ago though).
Theoretically, just adding this line:
spawnUnit: STR_HOLODRONE2_TERRORIST (or whatever you call it)
at the bottom of your parent unit, in the "units" section, should be enough to make the whole thing work (well, you also have to create the child unit of course).