It takes way too long for the average startup to be successful, and the overwhelming majority of them aren't successful enough to provide the economic freedom you are referring to.
If you can love the chase of a startup, and love the thrill and long hours, great!
If you are doing it so you can have "more free time later" there are many many better strategies.
Then when you're sick of it, move to a cheaper area, get a part-time job and sell 3-4% of your assets every year to make up for the deficit. If you're lucky, the safe withdrawal rate of 3-4% will even be enough (or close) to have the ability to quit working for a living completely. Or perhaps your part-time job can cover all your expenses and you can just keep letting your portfolio grow at the expected long-term rate of 6-7% per year.
But obviously, your total spending will increase, so you either have to earn more after having kids or save up more before having kids. If you want to raise your children in a high cost-of-living area, this will of course be even more pronounced.