score:8
Most airlines have a bit about this on their site - for example, on RyanAir, for travel in the UK:
Adults:
Any photo ID which matches the passenger name in the booking.
Infants and Children:
Children under 16 years of age (travelling with an adult) on UK domestic flights can travel without photo identification.
So no, they don't need ID to travel in the UK, as long as they're travelling with you.