Remember that she's playing with career ideas in general. Don't feel you need to take SAHM any more or less seriously than ballet dancer, firefighter or anything else she's come up with. I mean, you wouldn't seriously want her trying to become a professional ballet dancer, with all the body and psychological stress that comes along with that, right? But if she suggested it, you'd probably smile and nod, and possibly even look into ballet lessons, because you would know that it's most likely a passing thing. At 7, I wanted to be an archaeologist. By the time I graduated from high school, that idea held no appeal whatsoever
I can see SAHM being a bigger worry, because it's an easy position to just fall into by default, and can be hard to get out of without good support. But at this point, it's really still just an idea she's playing with.
If you're worried (which I wouldn't be now, but if it's still her life ambition come the preteen/teen years and you're concerned), you can seek out homeschooling or stay at home parent families with different structures to get her exposed to other possibilities. In my old homeschooling group, it was very common for one parent to work full time, and one to work a part-time job, or a job with a non-traditional schedule. In some cases, one or both worked from home. There were a few dads who were the primary homeschooling parent or regular attendees at park days. My DH is self-employed at home while I've been a full-time student the whole time we've been homeschooling so far and I will, at some point in the hopefully nearish future, be working 2-3 12-hour shifts a week (officially "full time", but with a flexible schedule that would allow us to homeschool even if DH had a more typical work schedule himself).
Or, as others have suggested, you can do this yourself. I kind of wonder how much of your anxiety about her future is rooted in some sort of dissatisfaction or anxiety about your own life. Examine what's going on in your head (financial stress, feeling you haven't accomplished enough of what you want in life, fear you're not contributing enough to society, not sure what you'll do after the kids are grown...), and see if there's anything you can do to address that. Find (or create) a flexible job, volunteer somewhere, find outside activities, take classes (towards a degree or just whatever looks interesting) at a community college - online or via some other form of distance ed if you like the idea of "homeschooling" yourself or can't otherwise work it into your schedule.
And, if it's still an issue once she's in her teens, you can be blunt about how being a SAHM is a perfectly worthy career, but the pay totally sucks, and it puts one in a very dependent position from which it can be hard to escape, so it's good to have some sort of other ambition or fallback plan. Just like you'd presumably be upfront about the downsides of any other career you were concerned about.