What type of bedding is safe for baby hamsters? Concerned about their well being...please help!?

We just bought a hamster a week ago from the pet store and it gave birth to 9 baby hamsters 3 days ago. Today I noticed that 1 had died and a little while later noticed another had died. Is this normal? We were using cotton bedding which I am scared had something to do with it.

organic cotton baby bedding