M.P.
You said that you've been mother to these children for several years now and your site says they're 9, 7, 6. I suggest that it would be helpful to them if you could focus less on the mom that they lost and more on the mom that they have. They need the security of knowing that you are their mom now. Their rightful mom.
That is not to say that you shouldn't honor their birth mother. I think Mother's Day is now the day to honor their mom, that's you. If they want to visit the cemetery this Mother's Day then it's good to do that but I'd plan to not do that next year. You all need to live in the present. You cannot undo the past tho you can feel sad and grieve.
I suggest it's likely that if given the opportunity your children are ready to let go of the grief and embrace their new life. This is a joyous Mother's Day for all of you. Focus on the joy!
I suggest that you find a way to let go of your guilt. It is hurting you and your children. Perhaps you could talk with a minister/priest or a counselor.
I empathize with you and think I understand why you'd feel sad and glad at the same time to be their mother. That is part of why you are a good mother for them. However, remember you are not taking their birth mother's place. You've become their new mother.