Acting Chairman of the Policing Board Brian Rea said they welcomed the arrests of 38 people to date in connection with the disorder but added:

"Members are also concerned that children as young as eight were involved in the rioting in Ardoyne and are fully supportive of measures the PSNI, alongside Social Services, plan to put in place to tackle this serious problem." http://www.belfasttelegraph.co.uk/news/local-national/board-backs-matt-baggott-on-ardoyne-riot-tactics-14890487.html
There you go Jim.
The number of young children involved was large enoughfor it to constitute a "serious problem" to the police.