Do you know if the daycare program is full? If the program is full and the staff are being offered a 50% discount, than the day care is losing money. If the spots that were being filled by discounted children were filled with full-paying parents than the school would most likely be meeting it's budget.
A very similar situation occurred at the Park District where I was a director of youth programs for many years. In our all-day program, there were three employee's children attending at a 50% discount. It was $500/month to attend, so the the program was losing $750 per month. Multiply that for 12 months and we were losing almost $10,000 per year. The all-day program was covering the losses we were seeing in the 3 hour preschool classes, so any shortage in the all-day program budget was affecting the entire early childhood program. In short, we couldn't afford to continue to offer an employee discount when the program was full. Therefore, employees had to pay full price for any class that has a waiting list. If there was no waiting list, then they could receive the discount. It was either that, or close the program and/or lay off staff.
While your situation may be different, keep in mind there may be a budget situation at play as well. Please don't make assumptions regarding the decisions management of the center had to make. There is often a bigger reason that the parents AND staff are simply not aware of. Believe me, I've been in management of child care programs and the perceptions staff/parents have of policy and procedure changes are usually way off from reality.
As far as the staff without children complaining that they deserved a raise because they didn't get a similar benefit is ludicrous because they don't have to pay ANY childcare costs at all. They already receive the benefit because they are childless anyway.