M.S.
I agree with those who said to be there for them, literally. Also, I think the most honest and heartfelt words are simply I love you. Just be there for them when they need you and if you are at their home, offer to help with other children in the home (if applicable), straightening up the house, even getting their outgoing mail to the post office. Many people forget to get bills and such sent out during grieving times and some just can't bring themselves to go out anywhere yet.
We have lost my grandfather, my husband's grandma, and his aunt also in the past 3 years and I think what helped more than anything was for us to help the families keep some normalcy. For example, I know my mother in law could not possibly think straight much less want to leave the house to get her bills paid when her mother passed so we ran errands for her to get them done for her.
Just let them know how much you love them. Love helps us get through difficult times.