My children should take my last name instead of my fiances and I'm going to insist on this.
What makes you want them to take your name instead?
I'm doing this to build myself a legacy.
Many families choose to use the mother's last name for the kids, rather than the father's name. You should talk to the kids about this so that they have a say in how they are named.
I just want to leave my legacy.
It's okay to ask if your children could take your last name, but you should speak with your fiance and your children first. If there's a big problem and your fiance disagrees then you could perhaps do shared last names?  That is, your children could have both of your last names, hyphenated.