A crematorium service is only about half an hour, so I think that your DS would be ok. But do have a plan B should he get fractious - perhaps someone who would be able to take him outside.
You wouldn't believe how unfazed children are about death and dying - a lot of 'perceived problems' are invariably due to the fact that most adults are uncomfortable because of their own issues surrounding death & dying.
I was 42 1/2 when DD was born; consequently she has known 4 deaths in 2 years.
She was 3 when my grandma died - I explained to her that she had died because she was very old (94) and her body had stopped working. If you keep it simple then they don't get mixed messages.
DD didn't come to the funeral only for the fact that it was a full blown catholic service. But she was at the wake, and my mum & I take her to the grave regularly (to pay respects & water the flowers!)
My father died when she was 4; he didn't want a funeral but she helped us to scatter his ashes. She was able to talk about great grandma being buried and by the end of it all, understood that cremation was another thing that happens to your body when you die.
My father had a brain tumour, but died of other complications before we could find out whether it was cancerous or not. However, it helped DD understand that when bits of your body stop working - especially important bits like your heart - you can die; because a year later her uncle had a massive heart attack and died. My EH refused to let her go to the funeral - but that was because his family have huge issues surrounding death.
My sis-in-law died earlier this year and was cremated. But she wanted a service so DD (6) was able to see it first hand. We did explain that people would be sad and why. We were all in the family cars together, & I did the Eulogy, so DD was very much part of the whole thing.
She was absolutely brilliant, going about patting people on the arm if they were upset, then entertaining them at the wake afterwards.
Obviously, I don't have any issues about death & dying - it's a final act of living and should be a celebration
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But I fully understand why people find it very difficult to cope with death. And struggling to do the 'right thing' where DC are involved.
Hope that this has helped you and others who may be reading.