Quit the game without saving.
If ever you saved once during or after the accident, go to your saves folder, delete your latest save (something.sims3), rename your previous save (something.sims3.backup) by deleting the .backup in the title. Reload your game. Your toddler should be safe.
If ever you saved more than once during or after the accident, and you don't have any backup, I'm afraid that there is no way to get your toddler back. Changing a sim's age back to toddler in CaS is glitchy in my experience, you may end up with a monster sim, don't do it.