What Is Expected from the Relationship?
This is a good question to ask yourself no matter what the relationship. Are you in it for fun? Do you want a long-term relationship? Do you even know? If this is a fling, be up front and honest; if you want a serious relationship, be up front and honest. Someone with a child needs to know what your intentions are in order to plan accordingly. If this is a fling, fine, but then you probably don't ever need to meet the children, it would just confuse them and open the door for a lot of uncomfortable questions. If the relationship is heading in a serious direction, you should be sure you are ready to be a potential stepparent. Regardless of the type of relationship, it is important that your expectations match your significant other's.
What Does the Parent Expect?
Does your new boyfriend just want a fling? Does he want a new mom for his children? Does he see you as his future wife? He should let you know as soon as he knows. If he simply wants a fling, all you need to worry about is not meeting the children and making sure no one accidentally stumbles upon the two of you in any compromising situations. If your relationship is just a fling but he insists on introducing you to his children, ask him what they will gain by meeting you. It really won't benefit them in any way, unless of course he has changed his mind and is looking at you as more of a long-term relationship.
If your boyfriend is looking for a replacement mom, talk to him about this being an impossible task. You will never be Mom — maybe a stepmom — but not Mom. His children have a mom; you need to respect that and help him respect that if he doesn't. His ex could be the most horrendous person in the world, and he could have sole custody, but, at the end of the day, she is still their mother, and you are not. You can model appropriate adult behavior for them and step into the role of a stepmother, but you cannot make up for mistakes or misgivings of his ex.
The belief that you can change a partner in a relationship is troublesome enough; if you think you can eventually move into the number one spot in your partner's life if you “put up with” your partner's children, think again; once a parent, always a parent. If you are determined to be number one, you should not be a stepparent.
If he would like to be in a long-term relationship with you and wants you to be involved in his life, you need to tell him where you stand. If that is not what you want, if you don't like children or have little interest in getting to know his children, you must tell him. It is not fair to him or his children to be involved with someone who does not plan on being around for very long. If you also would like a long-term relationship, then you can move on to meeting the children!