While it is always ideal to have ample volunteers to help plan, set up, and run an after-hours event, this may not always be possible. If you can get volunteer support from staff members and families that is wonderful, but if not, you may need to plan differently.
It is unrealistic to require unpaid work for these after-hours events. You should ask for volunteers, but do not be surprised if not enough people are willing to do so. It does not mean the employees are lazy or not dedicated, it may simply be too much. Consider a staff rotation of event helpers or designating a set number of events for each staff member to attend. Your options may very well be limited by extenuating factors, but being empathetic and supportive to your staff and their needs should be one of your top priorities.