Okay, it was my first time seeing this, but I started a new game and replaced the random starter Sim with a family I regularly play from my library. I aged down the kids one stage and clicked the chec...
That’s odd.. if it doesn’t happen again I’d say it’s an unlucky glitch. If it continues I’d maybe make a bug report, and might be best to save any Sims you make to your library just in case.