I'm thinking about moving the family to another town rather than trying a mod. The toddlers aren't aging now, and I think I'll keep running into problems if I try to keep this file going. I turned aging off, and turned it back on again, but don't know whether that will work. Anyway, would it be okay to move them? I've thought of a way to fit a move into their story.
Edit to add: Now the game keeps crashing at about 6p.m. Sims time.