If you are afraid, scared, our doubtful, then you aren't ready. Wait until you are ready. Tell him you need more time. I completely understand what you mean. I am not in the same type of situation, but I would do anything for my children, ANYTHING. 😁
Just remember, take it slow.
Ask for his understanding.

Definitely let him know. Ask him if something ever happened and you and kids were all close to his daughter if he would still let you see her. Just ask questions and express your concerns so he understands where your coming from.

@jaxxmommie22, I totally understand. Just make sure you lead with that, so he knows. My boyfriend now knows how I feel and he's pretty understanding. He says if I get a close relationship like that with his son and something were to ever happen and his son wants to see me or something, he would make it happen if it makes his son happy. You never know what his response may be. He might be afraid of the same thing.

@kdstiltz, thank you. I wanna talk to him bout it but I'm just afraid I'll say it in a wrong way that will make him think I don't want anything to do with his daughter. It's just hard bc i don't want the kids to hurt if this doesn't work out

I don't really have an advice but I'm in the same situation kinda. I was in a relationship with my daughters real father, he has full custody of his son and his real mom wasn't around. For over two years I raised that boy and fell in love with him. He called me mom as well. Once my daughter was born it was like a light switch and her father isn't involved and I left him. He refuses to let me have any contact with his son and it hurt a lot. I'm now dating someone with a child and he jokingly says "How's my daughter?" And all that kinda stuff and I'm afraid to get attached to his son for what I've already been through in a similar situation. I don't let it hold me back tho. I don't mean to make you more nervous about the situation at all, it's just my experience and yours could be 100% different. Don't stress too much, love. I'd voice your concerns to him though.

Pray about it. God will give you answers