Hmmm.. define "British" and define "child". The parents are jihadis who already rejected Britain and pledge allegiance to Islamic State and the new Caliphate they intended. There are people who are more deserving of sympathy than these criminals. Let them face the consequences of their decisions and regret their decisions for the rest of their lives.
They will be happier living in their own culture and religion rather than facing the bigotry and intolerance of the UK.
Citizenship laws are nuts in the West. A child should automatically given citizenship if it's born in the country of citizenship and at least one of the parents is a citizen. Also, get rid of dual citizenship. How can you be loyal to multiple countries.