My dad passed

This pass Sunday my dad passed at the age of 59. I am only 31 and I have two kids 6 and 3. My son is showing sadness but has not cried yet my 3 year old doesn't know what is happening but asks where her grandpa is. I am having a hard time but am pushing my feelings to the side because I have to take care of my kids and help my mom with the funeral arrangements because she is taking it really hard. I'm trying to help my kids understand but never had to explain death before to little kids. If anyone has any suggestions I am open to all ideas I want mostly my 6 year to some what grieve because I feel he is holding it inside.