While I have no doubts that you are a good mother to your children, I must say I disagree with your statement that nobody could replace the biological mother or father. Parents can, unfortunately, be guilty of taking their children for granted, and it is possible to permanently damage your relationship with your own children. I do not like saying so, but children are absolutely capable of feeling betrayed/unwanted/etc. by a parent.....and disassociating themselves from said parent.
And....if another adult figure in their lives fills the void, yes, they may very well accept that new person as their parent. Will the child ever think of the new partner as their "mother" or "father"? Likely not, but good chance they'll let them fill that role.
I guess my cautioning is, I'm very glad you have a good relationship with your children, but I'd warn new parents entering poly that, hey, you do this poorly enough, you can seriously f* up your relationship with your partner, your friends, and, yes, even your children.