In my opinion high chairs are unfortunately the most inefficient way to feed toddlers. After using them once with my first toddler, I've learned my lesson and never used them again. For toddlers I can control, I drag a platter of food from the fridge and place it on the ground right in front of them. Preferably in their room which always has one of those animal chairs for them to use. They'll grab a plate, toddle over to the chair and eat it. I always put a coffee table in their room too for them to place the empty plate. Otherwise they'll slowly toddle out to the nearest coffee table and put it there.
Now, if they don't have very high movements skills yet, that's not a bad thing, it helps to increase it. In that case, sometimes I purposely put the platter of food on the ground half way across the house so they have to walk a distance to get it. Not a good idea to do it that way though if their hunger is in the red. o.O
If I can't control any Sims except for that Gen's heir in, for instance, an I.S.B.I. challenge, as others have said, just have them pick up a serving from a platter of food and give it to toddlers.
I've read about using the buffet table before, but haven't tried it yet myself. I guess some Simmers also will get one of the bakery food cases from the Get to Work EP, fill it with food and place that for their toddlers to grab a plate whenever they're hungry. It keeps the food fresher longer too, rather than putting a platter of food on the floor and possibly forgetting about it until it goes bad. Having a Sim in the house with the Chef aspiration completed is great when having toddlers too. Just put a platter of food down and it never goes bad. Some also like to use the popcorn maker. Just have a Sim pop a batch and toddlers will get servings on their own when hungry.
If I still used the high chairs the one and only way I believe they'd ever work correctly is if the household contained a single parent and toddler only with autonomy turned off. Get anymore Sims in the house and it'll be a never ending in/out non-sensical frustrating game with the high chair.
Happy Simming! <3
EDIT: The only instance I can think where I made matters worse for my Sims when trying to help, was the same as someone else. I'll intend to drag a platter of food to the fridge for them and end up accidentally dropping it in the trash or dishwasher instead. Once they even ended up getting a trash plant outside because I had apparently missed the trash can altogether and didn't realize it. Opps! :D Other times I've thought it went in the fridge/trash/dishwasher and it's really stuck in the wall behind those things. After awhile it starts to stink and my Sims will complain and I can't figure out why without tearing the house apart to find the culprit.
Now that I think about it, that's happened with books too, which is very weird. I'll have accidentally dropped a book when trying to drag it in the bookcase for them and after awhile they start complaining about it. Who knew books could 'go bad.' I've read some stinkers in my life, but that's not the same. You can't smell the pungency of a bad plot. :D