Invisible Toddler?

@steepens1  Hopefully you've cleared the five cache files in your game folder in Documents.  (For reference: CASPartCache.package, compositorCache.package, scriptCache.package, simCompositorCache.package, and socialCache.package.)  It's also a good idea to go into your game and use "save as" (under a different name) to generate a spare copy of this save.  That way, as you're trying different approaches, you can revert to the original if something goes wrong or is otherwise not to your liking.

You can try a cheat-console reset on the toddler.  To open the console, crtl-shift-c and then enter "resetsim [firstname] [lastname]" without the quotes.  It's also possible that a "teleport me here" command might work, although I've never tried it with a toddler.  In the cheat console, enter "testingcheatsenabled true" and then shift-click the ground to see the option, if you have one.  (This is also a way to edit a sim in CAS, but you'd need to be able to click on him to do so.)  Be sure to turn off testingcheats ("testingcheatsenabled false") when you're done.

You could also adjust the age settings so that the toddler ages up soon and see if he shows up again.

Sometimes it helps to evict a family, save their house to the bin, bulldoze the lot, replace the house, and then move the family back in, all in one Edit Town session.  You can also try moving the toddler out of the house with an adult (while you stay with the rest of the family), playing for a day or so, saving, quitting, clearing your caches, reloading, and then moving them back in.  You can move in a spare adult in Edit Town if you'd like to keep the parents under your control.

It's possible that the kid just has a broken outfit; it can happen even without cc.  But it's also possible that the sim has become corrupted, and things may only get worse from here.  If you can't solve this problem, or if you start seeing other symptoms of corruption, it might be best to return to a previous save, from before he went invisible.