Tentative Offer Accepted, but Cost-of-living Concerns

Hello all, I received and accepted a tentative offer for a position in Sacramento, California starting at a GS-11. The position looks like a good fit for me, but the elephant in the room is whether my family and myself can afford to move to/live in California. We currently live in Oklahoma with two little children under the age of 4 (and a 3rd baby on the way). The position originally stated moving costs can be negotiated to be covered, which I fully intend to discuss further. Cost of living is fairly low in Oklahoma, but I worry that even with a two-income household, finances might not work out in our favor if we move to California, especially since we will need child care. My questions:

• What kind of adjustments did you/your family deal with moving to a higher-cost of living area for a Federal job? • What advice can you give in the negotiation process to help with this adjustment? Higher step? Is there a moving package or contacts to help with establishing living in a new state? Or any online resources? • If we determine that ultimately we cannot afford to make this change, can I back out before the final offer?

Thanks in advance for any advice you have!