If you doubt my statistics, refer to MSD's from earlier years:
At September 2016 there were 179,732 children reliant on a main benefit - slightly up on the June and March quarters but down on Dec 2015. This represents 15.3% of all children (aged 0-18) but in the 0-4 age group, the figure rises to 19.3%.
Sixty nine percent rely on Sole Parent Support (though this is an under count of the extent of sole parent dependence given thousands more receive jobseeker, emergency maintenance, supported living and youth benefits.) Most children reliant on a benefit appear in the child poverty statistics (though not all). Therefore, the main driver of child poverty and hardship is sole parenthood.
(Hat-tip to whoever submitted the OIA request from which the last graph stats are sourced)