It's unlikely - there's not much we've not come across before, but we've never heard of a case where both parents have died, and the tragic event occurred so recently.
I think that it is because this is so rare (and so tragic) that most people are probably reading this thread and thinking
"of course this kid should go to the local school with its connections / pastoral care /location etc"
I hope that the panel feel the same but of course it isn't as simple as that and the grandparents still need to produce evidence as to why it is the right school.
I suspect that much of this should be down the pastoral line - letters from GP or anyone who has helped the kid since the deaths, highlighting the need for further support. (I know if I was doing one for a case like this it would be very strongly supporting the grandparents and child)
Also is the primary school normally a feeder school? Are many of his friends going there? (isn't normally a reason for appealing but in this case maybe some continuity and familiarity may be needed - kids who know what has happened etc.
Are the grandparents taking anyone with them for the appeal - are you going?