The socialite is said to be a "strict" mother to daughter Harlow, two, and seven-month-old son Sparrow - her children with her fiance, Good Charlotte rocker Joel Madden - and is determined to only use natural products when she cooks for them.
She also applies the same principles to other products she buys for her family.
A source said: "Nicole is obsessed with doing everything she can to try to keep her children healthy and safe. She insists on everything being organic. The food, the diapers, the clothing. She`s always at Whole Foods.
"Her parents let her do whatever she wanted. Now she has strict rules."
Nicole - who grows her own vegetables - has also banned the use of mobile phones around her children because she is worried about the effect it could have on them.
The source added to Life and Style magazine: "She thinks the radiation can be damaging and won`t let cell phones near her kids."
Nicole has also revealed she uses chlorine-free, non-toxic and hypoallergenic diapers for her children, as well as taking other steps not to damage the environment while caring for her youngsters.
She explained: "We don`t use baby wipes - we use cloth with water."
Nicole previously revealed her priorities have changed since having children.
She said: "When you have the kids, everything changes doesn`t it? It sounds cheesy to say, but it does.
"Your whole life changes and your priorities change 100 per cent... in the best way possible."