Don't use a highchair, in fact remove it from the lot altogether. Have a Sim serve a meal and leave it on the counter. Call to meal. The Toddlers are able to grab food for themselves, especially when called. I play with autonomy on, since I play with large households. Louise Cantrell, is the worst mother I have ever had. It's surprising her four older kids turned out so well. Adrian came as a surprise. Perhaps, she's burnt out. I do recall feeling overwhelmed (going through that season women go through) that my older two kids were in their Teens, or just about there and I still had one in grammar school. I think it's a nice way to get a female through this change. I no longer had baby-fever. I almost didn't want to be Mom anymore. LOL. Kids are all fine, no worries. That youngest went on to serve his country in Iraq.
I don't do what was suggested by removing autonomy for a specific Sim, but yeah. It can be hard. Thankfully the four eldest in my game are all Teens, and they see to the Toddler for the most part. Phew. He just aged up. Yay! Another thing that does help, is giving Sims the Family Trait. I've discovered these are far more committed parents. Hang in there, Toddlerhood isn't forever.