Is it safe for me to open a savings account if I owe back child support but have already set up garnishment of my wages and I have a hearing in May for a decrease in current payments?

If by "safe" you mean protecting your funds so they aren't seized to support your children it depends on your State. In my State - NY - a garnishment which is current stops any other collection practices BUT I've seen Motions requesting that bank accounts also be liened when there is a large amount overdue.

The ongoing case has nothing to do with the savings account unless it's a hearing for arrearages.