I divorced my ex-wife for six years, and the child was awarded to me. She hasn't been to see the children once in six years, and my present wife is also very kind to the children.
Your mother will always be your mother. You should tell the children correctly that both of them are mothers. One of them gave birth to you and left, but they are also very kind to your children. One is your mother now, and your children need more love. Also, both of them are mothers. Don't go to court if you want, but it's agreed in advance that you can't instill bad ideas in your children, let alone destroy their existing families.