There is no requirement to show your passport or ID at security, whether travelling internationally or domestically. All passengers pass through the same security search area and security have no interest in where you are travelling too, just that you are travelling somewhere. Checking ID documents for international passengers is the responsibility if the airline (thru handling agent). Some airlines don't require ID at all for domestic passengers.
It is concerning that the boy passed unnoticed through three stages of control undetected, but I don't think any single person or group can be blamed. It was unlucky and certainly undesirable, but I highly doubt it would have happened had it been an adult and less easily bunched in with a group of low risk kids.
It's also worth noting that in Europe there is no requirement to check boarding cards in the aircraft (unless bound for the UK) so UK controls should be more likely to detect these things.