If it is wrong to depict Christ's body on the cross in a crucifix (because He has been resurrected) then is it okay to depict Him as a child in the manger? He is, after all, no longer a babe in the manger.
These depictions are fine because both the manger and the cross tell the story of our Lord. The manger reminds us of His humble birth and the remarkable story of the Nativity. The body on the cross reminds us of His sacrifice and suffering for our sins.