By the time the kid is into ICU, it's too late for Ivermectin to have a statiatically significant effect.

The kid is dying from the aftermatch of getting damaged by a flood of viral replication its immune system failed to stop soon enough.

By about a week into symptoms, the virus is neutralized by antibodies for nearly all people, even those with severe, worsening complications. It's the sludge of dead cells and damage everywhere causing the problems.

It's like Tamiflu against Influenza. Give it early to prevent the viral replication to comit too much damage, or don't bother.