I only see resentment if you FIRE and your SO continues working unless you're doing all the cleaning, cooking, and kid work. Do you plan to handle all of the household chores or will you be sleeping in, hiking, and binge watching netflix?
I like living in smallish apartments for frugality and efficiency reasons, which makes maintenance minimal. I've been living with roommates and cleaning literally takes 10 minutes once every few days, so I can't imagine this being a source of resentment. In any case, I was planning on doing my share (half) of the house chores, approximately. I would also cover half the expenses. The way I see it, how I accomplish this is really my own decision. I would work on passion projects, or maybe remunerated part-time so I would keep myself busy.
How much do you like to work? Is adding a year or two of work a deal breaker for you? If she had 1/8 of your housing/fixed expenses, would she be saving the same percentage of her salary as you are (ie. is she just as frugal as you are)?
I don't think it's fair to expect her to work for long enough to cover her FIRE, and I think you should both aim to be FIRE by the time you have kids. As bacchi says, unless you plan to be a SAHP, and housecleaner, it wouldn't work well for long.
And what do you want to do in FIRE? Travel without her or the kids? Have great experiences? Or sleep in a hammock in your backyard?
I could easily work a year or two more, in fact I probably will, interceded with a short leave. I guess the issue is I'm trying to maintain separate finances and obligations, so that she doesn't get a free ticket, and keeps working for her own goals (we haven't been together for that long, she's younger than me, and I don't know how she will react).
Currently she is about as frugal as I am, maybe slightly less but I am pretty extreme. I don't know if she would stay frugal with my income, or if her lifestyle would inflate, since there is no precedent, but I have no reason to doubt her on that.
Why don't you think her working to cover her shares of expenses would be fair? I'm a few years older than her, and I have spent those years mostly working -- wouldn't it be fair that she does the same, while I enjoy a well-deserved break?
About what I want to do in FIRE, that's another problem. Ideally I would travel a fair bit, at least at first while I want to. In fact I scheduled some time to do so by myself before we move in together. But if I FIRE while she has a stable job, I'll feel "stuck" at home (or in the same city) waiting for her and I'd probably resent that. I'd most likely end up working in some capacity in that case.
If I made 10x what my SO did I would have a really hard time quitting while they kept working forever. It seems far more efficient to work one year for every ten they'd have to and then FIRE together.
In all likelihood I'll help her financially and end up working more than strictly necessary. The latter is true regardless of whether I'm with her or not. But I'd feel bad making her feel the FIRE, screwing up her career, knowing that this is not really her money and that if we don't last, she'll end up with nothing. I'm wondering if there can be a middle ground, where I do help more than my 50% share, but she keeps working and I also have the freedom to relax a bit with work and go on trips by myself. I'm not that thrilled with the idea of being fused together forever in every way for now.