Child line is not blocked in the default o2 filter. Its blocked in the opt-in filter, as is most of the net (it's a whitelist). This is clear if you use the o2

Child line is not blocked in the default o2 filter. Its blocked in the opt-in filter, as is most of the net (it's a whitelist). This is clear if you use the o2 URL checker yourself. While not agreeing with the filters, it seems that there's a large and vocal contingent who are being hysterical and getting the facts wrong. They are not helping.