First of all, my sympathies to you and your family. The passing of any family member or friend is very hard on everyone involved.
My son is 4 1/2 and we have grieved over many people in our lives that have died. I believe we have been to about 12 memorial services so far. Because of my son's age, we only attend the wake service with him. He isn't one to sit still through a church service, but we will get there.
We are a very spiritual and religious family. My son understands death, to the degree that a young child CAN understand it and we talk about life/death openly in our home. I believe that it is VERY important for children to be a part of this final part of life and to share in the grieving. They need this outlet as well and I don't believe that children need to be shielded from death.
Take your son to the service. And realize that his only memory won't be about Bapa lying in a casket. It will be about the laughs they shared, the times they were together, the talks they had. It will give your son closure and help him say goodbye.
Since this will be your boys first service, please sit down with them and talk to them about what they will experience. As others mentioned, the Funeral Directors are good at dealing with kids. They can give you the guidance you need. Also reach out to your friends that can help.
Help your boys understand that it isn't a scary thing, just something that happens to everyone and the person that has left is now in a better place with others that love them.
Good Luck to you.
D. in milwaukee