Thanks I'll try that...I haven't used the bleach yet. Do I simply add it on cotton in the box or the paint itself?

I've started teaching a class on acrylics but they have only used poster paint in the paste. Frankly I don't remember using tempera paints except grade school.
I cut an 18-hole egg carton in half and used the bottom wells covered with a wet piece of paper towel.  This I fit nicely into disposable plastic squares with covers. I filled each hole with a few colors almost half-way and put their names on the plastic boxes.  Each week we give the paint a spray of water. They have not dried but once opened the children complain of a moldy smell.  I just smell paint myself?  I don't see mold but I did read somewhere if I add a small amount of bleach to the water it will prevent mold.  For the past 3-4 weeks this has worked for us but I am concerned that I do not smell what the children are smelling.  Any advice?