@PlumDutchess I ran into something that may be a thing or not, as I was avoiding using the highchairs for maybe close to a couple of years, so I'm comparing to previous memories of using them...
When testing the build I just revised from a previous version, I found my Sim parent could put food on the high chair but could not put the toddler in the high chair. this high chair, which snapped to the table like the chairs do, was placed not using MOO against the table, and the back of the high chair was close to the wall, but not too close. However, as I said, it gave the parents a routing issue. Turned out that all I had to do was snap it to the table in a way that the parent could fully route behind the chair and place the toddler in. I tested further and it turns out that the parent can only place the toddler in from straight in front or straight behind the highchair. Is it possible your Sim cannot access the chair by straight in front or behind? If you stick the high cahir in the middle of the room with nothing near it, can they use it then?
If that's not the issue, do you have Mods or CC? If so, try removing them and the localthumbcache.package file and see if the issue remains.