I'm a parent of grown children, not young ones, but I'll bite. The short answer is, you can't really predict your expenses because you can't predict your needs. What you will need to spend money on (clothes, food, medical care), and what you will choose to spend money on (education, activities, entertainment) both depend on the number of kids, their state of health, their talents and interests (and the degree to which you want to support those), and the opportunities and costs in your area.
If you'd like one data point, I would estimate that at the peak of our child-spending years we spent approximately twice what we spent before kids. We had a larger family than most(seven kids), but also a simpler life than many of our peers: few paid extracurriculars, no high-level sports, no vacations other than camping or visiting family, no personal electronic devices, no orthodontia. I don't see it as selfish, but as part of bringing my kids up with the values I want to impart.
If you like your job, you might as well stay and add to the stash while you decide. You don't have to have 25x the peak spending, since you won't be spending that amount indefinitely. You can switch to part-time work, or you can retire and then pick up some paid work later on if your expenses rise beyond what your stash will support.
Congratulations on the stash and the baby!