If a miscarriage happens from zina do the parents still get guaranteed heaven

Nothing is guaranteed for the heaven except one own deeds that has reached to the lavel of acceptance of Allah (swt). But we dont know which deeds of us has accepted that is why we do all the recommended deeds again and again while we are living in this material world.
Zina itself is a greator sin therefore first of all both who has committed this act of Haram has to ask for forgiveness seriously from Allah (swt). Rest is depends upon His mercy. Because Allah knows better.