This is true …. usually when a mother is committed to prison for a ‘serious’ crime (like this one) any children in her custody are usually taken into care by the Social Services.
Mind you normally they would expect to contact the father – unless they didn’t know who the father was and the mother (acting out of spite maybe) refused to tell them who or where he was.
Her b/f (who sounds as if he had a lucky escape) probably doesn’t want to know and may have just felt lucky to have got away with his life.
Could be that the Social Services have put the children up for certainly fostering and maybe even adoption.