One thing we've learnt from all the shocking cases of children 'known to the authorities' in this country and the UK who ended up murdered by their parents is that violent parents can be incredibly devious, and very convincing.
It's like sexual abusers, who can live outward lives of apparent 100% normality while wreaking havoc on children's lives, be it their own or other children's.
Why would anyone doubt a mother who turns up at a meeting with 'her' little boy? They will from now onwards, probably, but the idea of a mother presenting a stranger as her [murdered] son is so off the wall that it's not surprising that people just accepted it.
Social workers, as well as being hugely overworked, are in a 'damned if they do/don't' situation with child protection - they get flak if they fail to take a child away from parents who turn out to be abusive, and they also get flak for taking children away from parents who may turn out to be abusive.
It's no wonder so few people want to work in child protection, and that's causing low staffing levels and high workloads so children are left unprotected.. a vicious circle.