When I have one of my Sims "Load Dishwasher", all the plates on the table are collected, but the plates/bowls on the highchairs (from giving toddlers food) are ignored. The Sims can usually find stray dishes left all over the house by guests, but they ignore the dishes on the highchairs.
I have one mod: the UI Cheats extension. My toddlers just aged up to children, but I'll try with UI Cheats disabled once my infants age up to toddlers.
Edit By crinrict: Adjusted Title
