Strong marriages make strong families. You'll be surprised how much resentment will be extinguished from your children's hearts. Please don't turn a blind eye to the problems in your household. Your children should know without a doubt that you expect them to respect your spouse. If your spouse's ex is spitting mad that you have entered the picture, you're in for problems no matter how much the children like you.
Children are very loyal.
Maybe I'll sound harsh, but you're better off expecting tears, resentment, confrontation, defiance, and a tongue-tied and overwhelmed child who would rather be anywhere but in the middle of the family's blending process. Prepare yourself for these things and your feet won't be shot out from under you. Instead, aim for earning your stepkids' respect. Focus on being someone who has integrity and honor. As much as the children may resent you in the beginning, eventually they will have to acknowledge their respect.