No, I would not describe reasonably consensual sex of the sort required to make children as /hardcore/ porn. Hardcore porn probably requires strange ustensils, use of bodily orifices in ways that do not lead to reproduction, often blood, pain or simulated pain, obvious coercion.
Children, not having yet developed the sexual drive, do not understand the motivation for sex. However, I do not think that seeing normal consensual sex, which I would describe a porn but not hardcore porn, would be seriously damaging to children. But the violence, simulated or real, common in hardcore porn is very frightening for children who do not understand the world but know that they are weak, uninformed and defenceless. I would ezpect it to be seriously traumatising for a majority of pre-pubescent children,