Anything you receive of value from your employer in return for providing a service is taxable income, unless it falls into one of the categories of protected benefits. If you are employed by the union, then this is taxable wages. The only way it could be excluded is under a theory that this is "day care" for a dependent, that is provided so you and your spouse can work. And for that to work, the payment would have to be included in box 10 of your W-2, and you could cover the expenses that we paid for care while you worked (such as 8am to 6 pm) and not the full cost of an overnight summer camp.
If you are a member of a union but not an employee, then this is probably a non-taxable benefit. You pay union dues that are not tax deductible, so any benefits of being in a union are generally not taxable income to you.
