Oh Melissa dear, bless your heart, I am so sorry
. I know your heart is broken with the loss of your Poppy, but rest assured that he is resting in peace with his creator, out of all pain and suffering. His memories are tucked safely in your heart and will help guide you through all your ups and downs.
Far as your children, like aready suggested, a drawing or writing letters is a wonderful idea. Other things I've seen done at service, is to place their pictures with him around during service, let each grandchild pick out a flower and make a bouquet for his casket, maybe release balloons in his honor. All this makes them feel a part of his love on earth and now in heaven and is such good therapy for them.
Remember, he would foremost want you to take care of yourself and your little blessing. I send continued prayers for you and your family at this difficult time dear.
It was a terrible loss to all of us, including the grandchildren who had never known another grandfather. At the funeral we allowed them to put things in his coffin if they liked. One child put in Granpa's coffee cup, another put a statue of Jesus, one wrote a poem. My daughter didn't know what to put in there, so she just put in a brownie, "so he won't be hungry" 
