Maybe compromise. Let her pay 1/2. Or just cut your losses and get a new one.

When you let her go are you going to give her severance pay? If so, that may factor in to whether or not you let her pay for replacing the stroller.

She is being fired for cause, because she is a bad employee. This has nothing to do with who pays for the stroller. Holding her accountable for losing her employer's property isn't bullying. Accidents happen. If she damaged the stroller through ordinary use or it was stolen despite ordinary use and reasonable precautions on her part, I would pay the full cost. However, I can't imagine losing a stroller and not even noticing in the absence of negligence. I'd ask her to split the cost.