They can't feel close to you if you pretend everything's coming up roses. Roses may be in the picture, but first, acknowledge a few thorns. Don't appear overly nice. Walking on eggshells isn't fair to you. Your stepchildren need to see that you're a real person.
Normal people do have a full range of emotions.
As time goes on, keep reminding yourself that you're striving to make life comfortable, not perfect. In this process, don't forget to protect yourself and your new home life. The ex-spouse may be throwing a lot of rocks your way. Your stepchildren may think they have nothing to lose by hurting you. But you have a lot to lose by fighting with them.