How do I prevent little white pimples from forming on cornrowed hair?

My sister in law often puts cornrows in my twin daughters%26#039; hair. She is very good at it, can do designs etc. I like their hair like this since it is low maintenance for me, but they always develop painful white bumps on the scalp. How can I prevent this?|||Maybe she is doing them a little to tight. I did a girls hair once and I did hers to tight and she said they had never been that tight before, and that was what caused her to get the white bumps on her head. To hold the hair in tact and still look good use a gel that wont cause the hair to get stiff, but will help hold the stle. Hope this helpes!!!!