They definitely would have been arrested in the UK as we need only “reasonable suspicion” to nick someone, interrogate them and instantly search their homes and property.
Unfortunately you usually need more firm evidence than that in the US (“probable cause”)… and with no body, no blood, no cause, and no evidence of wrong-doing other than suspicion, the police are a bit stuck.
ETA: that’s not to say they could/would have been charged with anything in the UK, but at least proper searches of homes and vehicles could have been done before the family had chance to leave the campsite and hide any evidence.