Alright I found a temporary fix for this!
So when this happens I will press alt+tab (to switch the window out of the game), then I will press the Windows key + L (to lock out of the windows session without closing it). Then, I will log back into the session and re-click on the sims game. The sims start moving again.
It takes me about 10 seconds to do so, but the fix is temporary because at some other point it will bug again.
I also have to point out that it only does so in my 100-baby challenge house, perhaps because there are too many sims on the lot.