I'm not sure what you mean by this. Having a child at all is inherently an interruption in pre-child lifestyle.
Allow me to explain. Assume you and your spouse are DINKs (Dual Income, No Kids). Assume once again that you have a nice house in the suburbs (or a large condo in the city, just not downtown). Finally, assume that you're living on both incomes, yet still putting away 401(k) and IRA money. Now, if you have a child, you have two options -- someone stays home with the child to rear it, or you get a nanny after maternity leave is over. In either case, you're looking at an interruption in pre-child lifestyle, since you face the loss/reduction of one spouse's income, or the additional financial burden of a nanny.
If, on the other hand, you've been following the FIRE principles, you've lived on one spouse's income (or less), and banked remaining money earned. When you have a child, and one spouse quits working or you hire a nanny to help out, there is likely to be minimal impact on lifestyle (although there will most certainly be a substantial impact on FIRE).
Unless you're truly poor (which I think excludes almost anyone considering FIRE), the decision about when and whether to have kids should have little to do with money.
I disagree. Financial affordability should be a top priority in deciding whether to have kids. The last thing we need is another middle-class family struggling between paying for their 3-4 kids, and keeping a roof over their heads, food on the table, clothes on their backs, etc.... when only one spouse works and then loses his/her job. Having witnessed this first hand, and the financial fallout resulting therefrom, I'll never allow myself to get into such a situation. If that means I don't get to have 3-4 kids, but rather only 1-2, and then only in my mid-to-late 30s, so be it.
However, I don't see why it should take a decade or more to save up a bit of cushion. Despite all of the Hollywood actresses having children in their 40's, fertility can be a real issue, especially those who want more than one child.
It shouldn't take that long per se, but in today's competitive economy, you have people coming out of graduate school in their mid-20s with $40k+ in debt (sometimes $100k+), who don't get into a financial position to even consider having kids for 5-10 years, even if they get married right out of school. As for the fertility issue, if the wife is at least 5 years younger than the husband, then such a circumstance would be more or less ideal for having kids and the financial ability to afford them without an interruption in lifestyle.