C.S.
You just tell them. They deserve to know. Now that it is the weekend, it is a good time to say something. I think the 10, 8, & 6 year old are old enough to tell. Chances are the 2 year old will not understand. I would not do more than let him/her give grandpa's photo a kiss and tell him/her to say miss you or something. That way, they will grow older knowing his photo, as well as knowing he is no longer with them.