Hello resigned,
Is there a pic of mom with your kids for them to have in their room? It might be nice for them to have one there so if they want to look at her or think about things without 'hurting you' - they have a bit of privacy that way.
Some days we just can't fix it. You can't 100% fix the lack of their Mom in their life. I am really sorry. My RAH lost his father as a child and it has definitely played a role in who he is as an adult. I think acknowledging her and being honest is the best way to go - gently.