I think it has to be an informed decision by the child.
Ever-so different, I was 21 when my mum died, and of course being an adult, it was expected of me to be there.
I had never been to a funeral before. When I saw the herse pull up outside our house, I ran to the bathroom and was adament I wasnt going. There was something I just couldnt put by finger on, telling me I shouldnt go.
After DH being supportive, I eventually got in the mouners car and went. I had a complete breakdown during the service, still cant recall anything other than being on my knees in tears. I look back now and wish I hadnt gone, but also feel that if I hadnt gone, I would wish I had.
In some ways I now regret not seeing my mum in the chapel of rest, but at the time, with a 2 week old baby, I wanted to remember my mum alive, not lying in a coffin.
Sorry for the ramble, in my opinion, if the child can be told exactly what it will entail (which I had not been told about) and wants to go, then she needs to be allowed.