Ask HN: Evening freelancers, how do you manage your time?

So I tend to have a fair amount of time in the evenings, but due to a fairly erratic childcare schedule I never really know when I am going to get the most of my freetime in.

I would like to do some freelancing in my spare time with a view to quitting my job and doing it fulltime once it is up and running. I am worried that clients aren't really going to like the fact that I will not be able to dedicate a predetermined number of hours per week to their project and will only be able specify a vague 10-20 hours per week timeframe.

Do you folks have any advice on how I can best handle this?