With this strategy in place - then deal with your stepchildren's emotions. This is something that will be dealt with over a long amount of time. Your stepchildren ultimately have to come to their own conclusions about you. Depending on their age, it may not be such a long haul for them, or for you. When you are with them, let them know you care about them and just be yourself.
Kids are clever beings and can usually spot a bad person quickly.
If it seems that your stepchildren have rejected you - leave it alone. Don't take it personally. If the ex is meddling in your relationship with the children - you won't win by trying to win. Let it be. Be yourself and let what comes around - go around.