I ended up in a managerial position at my company somewhat against my wishes due to various circumstances and I don't know how to reconcile this with my leftist beliefs. I do what I can to give my team raises whenever possible, and encourage everyone to take as much time off as possible, but I can't help but feel like a traitor to the cause. I have a mortgage and a kid on the way so leaving this job isn't really an option, but how else can I be a ally to the worker movement?
A manager is the filter between workers and the capitalists. Your job is to make sure the shit falling down stops with you. Do not let the workers come into contact with the elder gods upstairs.
Also, if you can, encourage leftists and union talk, or at least turn a blind eye.
If you don't have hiring/firing privileges, you can generally join a union still. If you do, many unions have associate groups for those who are sympathetic but don't meet the criteria for being a worker. For instance, the IWW has it's General Defence Committee as a way of allowing active support while air gapping managers from organisers.