Sim toddler regression

Try to force error and reset him. Type: "boolprop testingCheatsEnabled true" ,
Shift+click the toddler, select Force Error and Reset. If that doesn't work then you have to either get rid of the corrupt toddler or wait for the social worker to come to take all the children off the lot. To get rid of an underage sim you can use the Tombstone of L & D (turn on testingcheats, shift+click any sim, and spawn the tombstone), spawn an adult or elder sim, and have the sim move out with the toddler, then whenever you're next in the neighborhood zone you can delete that family. After you get rid of the corrupt one, you can adopt again or spawn a replacement toddler. There's another option that's a little tedious; use the MaxMotives cheat. Open the cheat window and type: MaxMotives. Keep doing that when the toddler's needs get low until it is the toddler's birthday and it grows up. Growing up may correct the problem. Speaking of growing up... You should be able to grow up the toddler using a cheat, with the testingcheats on shift+click any sim, select: Spawn... , select: More... , and select: Sim Modder. Click on the baby doll that appears and choose: select Age. You can make any sim any age you want. Be careful about Aging an Adult back to a child. For some reason they have an F in school and the social worker comes.

Force error might work. This has happened when I put a female sim in the guys bathroom she just stood there.

I THINK I FOUND A FIX TO THIS!!
Bring up the cheat box (ctrl+shift+c) and type in:
boolprop testingcheatsenabled true
Then shift+click on a sim -> Spawn -> Sim Modder (a fake baby will spawn).
Select the regressed toddler as the active sim, click the fake baby/Sim Modder and choose Set Age -> Me(active sim's name) -> Toddler.
Your regressed toddler will play the animation as if it's aging up but it'll still be a toddler.
The only downside to this method is the toddler will have a "grew up badly" memory. I can live with that, I've GOT MY TODDLER BACK!
Make sure to bring up the cheat box after you've finished, and type in: boolprop testingcheatsenabled false
To turn the cheats off cos apparently they can mess up your game.

[My backstory if you're interested; my family hired a Nanny cos both parents work and can't be home to look after the toddler, but one day after my sim dismissed the Nanny, the Nanny picked up the toddler before she left. The toddler stuck to the Nanny and rode off in the Nanny car with the Nanny?? I saved and exited to neighborhood and went back into that household and the toddler had come back but had regressed to a baby just laying on it's back next to the mailbox. Whenever my sim tried to pick the toddler up a diologue box came up saying error and I kept pressing the option to reset my adult sim it did reset her but I still couldn't pick up the toddler.]