WhatsApp not downloading on child's phone

I want to put my 11 year old on the family WhatApp chat.

We have family sharing. I have set his Family sharing restrictions to Allow All Apps but it still will not download - we get the message, Due to restrictions set for this device, this app cannot be downloaded.

I realise WhatApp is age 12+ but we feel able to monitor his use of WhatsApp and it will be useful for us a a family to have him on the app.