Children under 16 travelling with an adult on UK domestic flights (and UK-Republic of Ireland-UK routes) do not require photo-ID.
Children under 16 years (including infants) travelling with an adult are required to have photo-ID when travelling on flights between European Economic Area (EEA) countries. Acceptable photo-ID for children under 16 is either a valid passport or a valid National Identity Card issued by an EEA country.