@Mistressinthetulips Yes, that was my son. They hadn’t learnt about World War Two in primary school.
I echo other posters recommending to email Head of Year and not the parents. Or even, as I once did, write a message on the group itself telling odd the boy in question (this was in Year 6 when one of the boys was sending inappropriate photos in the WhatsApp group) all I got was, ‘Woah! Rusty’s mum is triggered!’
Children under 10 are using these apps, the age limit means nothing. You want to try checking out the Friendlist of your child’s fellow Instagram classmates, the girls are already posing in that hand on hip, finger in mouth pose at age 8 
I’ve removed him from Instagram because of that, although he was only using it to garner subscribers for Roblox or some game or other.
I found with the classmate/butterfly knife issue, it leads back to the parents ultimately. His dad is on social media posing behind his son the pair of them holding their knives. Who knows if they’re ‘trick’ knives or not, fact is the boy brought it into school. With parents like that no doubt the boy is on a safeguarding radar somewhere anyway.
If you choose to give your young child a phone, you have to police it. If the kid doesn’t like it, tough, they don’t get the phone. There’s absolutely no other way of monitoring their behaviour.