You should understand that your partner’s children are and should be their number one priority. If you want to be with this person, it’s something that you need to respect, support, and cultivate.
Do your best to gradually build a connection with your partner’s children. Ask your partner for help and advice to let the child know that you are their ally and not a threat. A good idea for the beginning will be to learn what hobbies and interests their children have. For instance, if the child likes baseball, you can get tickets and go see a minor league game. Or, if the child is into Disney princesses, buy them a Disney princess doll.