B.B.
I tried for quite a while with my son. I just had a brand new baby and didn't have the energy to fight him anymore about it, so I finally told him he was going back into diapers until he decided to be a "big boy." I made him go down for naps when his baby sister did and he didn't get any of the privileges that a "big boy" got. It took about 2 days until he decided he hated diapers and didn't want to be a baby anymore. Also remember that new babies (especially premies) add a lot of stress in the house, not only on parents, but kids too. Good luck!