This is so sad, I cannot even imagine their pain. The idea of a keepsake box, the poem, planting a tree and the stuffed animal are all touching thoughts to me.
But, robinrs' post brought tears to my eyes. My DH's father died suddenly and fairly young (50). My DH, to this day, talks about the fact that after the initial mourning period of one month, only two friends continued to call him, let him talk, cry, or simply sit quiet with him. Everyone else simply stopped calling and, he felt, avoided him because it was too uncomfortable. He would fight to the death for those two friends who were always there for him them during that time; they proved their worth and he could not have gotten through it without them.
I whole heartedly concur with Robin's post about just continue to be there for them in the coming months and even years. That will be the best thing you could ever give them.