The custom at one museum in which I worked was to offer the staff kids "free if space available," the other was "half price if space available."
In neither case were discounted/free kids allowed to take places for which the museum could get full fare.
I noted in an earlier posting that the writer was not sure about profitability of classes offered. Please, ask your finance folks to help you determine profitability, and know what you are dealing with. An informed staffer is a greater asset to the organization!
A rule of thumb (just to open the conversation):
Compute all income from all sources (T-shirt sales? Lab fee?)
Compute costs (materials, staff, equipment rentals and overhead. Not sure what your overhead rate is? Rule of thumb time again: about 30% of your staff's salary for the period they are being paid. Includes planning time and clean-up time; any time charged to the project, add 30% to cover overhead. Covers computers, utilities, benefits, janitorial, office supplies, insurance, etc., etc.)
When at one museum it became necessary to raise class costs, staff became incensed that staff salary and overhead were charged against the bottom line. Yet it is well that the staff understands what it costs to operate. And it makes budget time a lot easier!