Honestly, I think it's wonderful. His children can be a wonderful chance to see how you will cope with your own children in future. The main idea to become a friend for his children...not mother, sister or dad's new wife but a real good friend. It is very important when child trusts you, there some things children will never tell their parents, but they share all the secrets with their friends. It is very important to be a resonable friend, who can help with different issues and whose advice will be both useful and won't be harmful.

Also it depends on how old his children are, because it's always easier to become a good friend for little ones, but it is way harder if his children are teenagers...I remember myself being teenager...Gosh, I am so greatful my parents went through all those days while I was a complete disaster...anyway, you can try to show that kid that you are not going to take all father's attention, you are here to help him to make family life easier, happier and more entertaining...You are the person who can listen, understand and help...not the one who doesn't care about them. And maybe it will take a lot of time, or it may work at once...Anyway you should never give up, because they are children of the man you love!