I just had that happen to me, both the kids were at home and if said they were at school. I made a sim travel, too, and that fixed it.
I went into Manage Worlds before I loaded that game (they were at school at the time), then when I loaded it, my sim was standing out the front of his house with a bunch of other sims, in two rows, just standing there.
The weird thing is, my sims are on full autonomy, so even though I couldn't click on them to do stuff because they were "at school", they were doing things. One of the kid sims did his homework, and I got the popup saying homework complete. But after I travelled and got their icons back to normal, his homework wasn't started.