It really depends on the age of the children, IMO.
I had someone who came and helped during the day when my children were preschool age. I also did a lot of work after they went to bed at night. Fortunately, younger children tend to go to bed at an earlier hour.

Bedtime at our house was 8 p.m., do or die! I then had a couple hours to bang out as many lines as I could. Even with a mother's helper, I had one child who needed a lot of attention from me and many times I worked with him in my lap - where he was allowed to stay as long as he was quiet and didn't wiggle too much. I was able to keep working and he was more than happy to just sit quietly.
I can't imagine you could really get many lines out when you have small children demanding your attention but you can work your schedule around theirs so you can be very productive during some hours and semi productive during others. And I highly recommend that a helper, even for a couple hours a day, can make enough of a difference in your productivity that it offsets the cost.