Assuming MO has a "head of household" exemption (I don't know), you would file something akin to a "claim of exemption" with the court that issued the garnishment. Odds are the court's website will have such a form online.
I found the form below from the MO court website. But you should do your own search.
In Eastern District of MO, where I practice, I would use a different form than the one Attorney Berkus cited, but that one could work. Check with your payroll department, it should be able to confirm
P.S. MO does have a head of household exemption.,