Thank goodness your friend has such a thoughtful and present person in her life. You are doing all the right things and, no, it will never feel like "enough" because nothing can fill the voids she has experienced. Please don't minimize your efforts because being there for her and listening helps her bear the grief and still feel connected to the world.
It is hard to hold someone else's grief, so be sure and take care of yourself too by having some relaxing/pleasurable outlets for yourself, and encouraging involvement of other friends or family with her.
I am concerned about the children as well. They need to know that their world has not collapsed. They need their questions answered and they also need to be listened to when they talk about their little brother or anything else. Perhaps you can help your friend organize playdates for the kids, or go along on a family outing to the zoo when they are ready.
It can be very helpful for your friend and for the children if they can think of ways to remember those they have lost. Something like planting a small tree for the loved one, or making a memory book together; finding ways to celebrate the loved one's life.
Lastly, you can also help by checking in with your friend to see if she is slipping into depression, or if the kids start having behavioral issues. Sometimes professional support, counseling, or therapy may be called for.
Best wishes, Jessica T.