It seems that Sims have always been bad parents to babies and toddlers. It's hard to get them to do what I tell them to do most of the time but it's way worse with parenting tasks. Like with feeding a toddler. One will pick the child up and sometimes hold it for awhile then put it down then pick it up and do it all over again. Then when the child is finally put in the highchair, one family member or nanny will take the child out of the highchair and another will put it back. This goes on for awhile sometimes the child steals some food off the table and wonders off to sit on the floor and eat.
After this last update it's now even harder to get the family to take care of a toddler. At first it was only a problem with feeding but now it's with all child care.
I know exactly what you mean and it is easy to understand : you are fighting the autonomy option located at the game settings. In previous games if a certain sim is interacting with something and another one wants to use the same thing they would wait for them to finish before trying to use it, now you can tell a sim to feed a toddler and as soon as you sit him on the high chair the second one would pick it up to play. The sims 4 is the only game where your sims don't do what you tell them to and it gets annoying if you tell them to read a book to increase skills and they suddenly stop to watch TV or dance.