seekingcalm, you have every right to express yourself to your BF regarding the situation,
expressing how you feel can be very theraputic, the dialogue can be very simple, I feel like this ........ when this happens......just express your thoughts. This is one of those touchy situations, If you say ....... YOU make me feel........ or YOUR kids make me feel........ that is opening up a whole different can of worms. You can only speak for yourself.
I can only speak as an outsider, my ex-husband has had a (few) SO in his life. Our adult daughters that range from 24-37 have chosen to be polite, but not interested in pursuing a relationship with the woman. Why, I cannot not tell you. It is their choice, they are adults. I know they have family dinners, I know they call their dad when they need advice, they will meet for weekends at the cottage, but they do not have a personal relationship with his current fiance.
I can only speculate that it has something to do with their relationship they had with their father long before the SO appeared in the picture.
So don't beat yourself up about the situation, it may very well not have anything to do with you. All we can do is put our best foot forward.......Wishing you the best