I was 22 months refined sugar free when I was diagnosed and I have to say that the fairly clean diet did appear to make it easier for my body going through chemo, unfortunately the hospital food was the worst and I had to relax my personal comfort levels everytime I was admitted. I'm not perfect and think that whole foods 80/20 rule is pretty realistic because some nights you are so stuffed that dinner is going to come out of jar or a box. But like Robyn said don't beat yourself up about it if you slip up.
I've got children as well and they don't eat as cleanly as us but they are improving. They are learning to substitute the heavily processed foods with better options and to make things from scratch themselves so they know what is going into it. We play a game in the supermarket when they are choosing lunch box items, they have to find things with the most real ingredient, words that they understand on the ingredient labels, minimal or no numbers/scientific names.
Keep persevering as some improvement has to be better than none at all. A couple of websites you might want to try for inspiration:
http://draxe.com/section/recipes/
http://www.100daysofrealfood.com/start-here/
https://iquitsugar.com/recipe/
There are also the Organics Sisters on facebook and another ok site is Mind Body Green