Get rid of the high chair!!! Put a little kid chair - the little bear chair - next to the table (not facing the table, as they won't be able to sit in them), and put a plate or serving dish on a counter, and have the toddler grab the food. They'll sit in their little chair while the rest of the family eats at the table. And they won't throw the food, either.
Seriously - getting rid of the high chair eliminates so much trouble, you'll have plenty of time to teach them and play with them and keep their needs in the green.
When they coded it, I think they gave it too many options. IIRC, with Sims 3, you could click on the high chair to either give the child a bottle or give the child food, and the only food was a bowl of mush. It was all one interaction. Now, putting the child in the chair is separate from the 'give food' interaction, so the toddler can queue up a 'let me out' interaction in between, or another adult can queue up a 'take toddler out' or 'give food' interaction in between putting the child in the chair and giving the food.