They may be excellent at getting into their zone, I don't work very well in snippets of time. I need a calm quiet stretch to get into the right frame of mind. I assume the volume of writing achieved may also be related to the age of the children too.
The littlest ones can be so exhausting and I think you are in a similar boat to me with ages Rebecca. My littlest is only 3 so too old to be napping but too young to understand that I need some time when I am not being disturbed. I also don't feel able to focus my mind on getting into the story while I am responsible for the children I always feel like I need to be on alert.
Ideally I would like to find a way to snap my brain quickly into writing mode so that I can by-pass the news/random internet wind down phase that comes after they go to bed and get straight into it for a couple of hours.