Is it possible to start a PhD at 36 without taking a huge hit financially?
I did my PhD in computer science in the Boston area. Through an RA and a bit of TA, I had my tuition, health insurance, and conference travel expenses covered, plus a stipend which has since risen to $36,800 before tax per year. (With a bit of googling you can look up what financial support various PhD programs offer, as well as health insurance costs for dependents -- for a large family, this might be $10,000.) That was enough for a modest but comfortable single person's lifestyle in expensive Boston, but I would struggle to support a child or mortgage with that. PhD stipends will vary by field (i.e., how much funding professors have) and by university (i.e., local cost of living).

You will know your financial situation and it's up to you to judge whether you can cut your expenses and/or dip into your savings to make that work. PhDs are getting longer, so keep in mind that you probably need to do this for about six years.

There are a few other options to make it better financially:

Ideally you can work something out with an employer where they will pay you while you do your PhD, probably in exchange for some part time work or a promise that you will continue working for them afterwards. Such an arrangement is more common for shorter programs like an MBA. 

It's also possible to spend some time during the PhD working in a company that is somehow related to your studies. For example, a lot of computer science PhD students will do internships at companies like Google or Microsoft. A 10-week summer internship could give your annual income a >$10k boost.

If it all gets too much financially, you can take a year or two off your PhD to work and then come back to finish. That's not ideal, because it interrupts your work. However, it's better than needing to drop out entirely.

Overall, however, I think it's unavoidable that a PhD will entail a financial hit. A PhD is almost never a good financial decision, but if you are passionate about research it's definitely worth it.