In the name of Allah the Beneficent the Merciful
All praise is due to Allah Lord of the worlds
May Allah bestow his blessings upon Muhammad and his Progeny and curse their enemies from this day to the Day of Judgment.
Assalam alaikom,
1. A Shia is one who obeys Allah and carries out His injunctions. Imam Al Sadeq

says: "By God our Shia [followers] are none other than those who obey Allah".
In the context of the abovementioned tradition, however, a Shia is one who believes in the pillars of Islam, including the leadership of the 12 Imams. It would be illogical to say that only practicing Shi'ites are meant by the tradition because practicing Shi'ites do not burn in the fires of hell, period. Only those Shi'ites enter hell who do not practice what they believe and will have to suffer the consequences of their actions either in this world, or if their bad deeds greatly outweigh their good deeds, they must be cleansed in the subsequent worlds (such the Barzakh and the hereafter). Justice is, thus, served by means of punishment for the bad deeds.
2. The verse you refer to is:
Quote
انما التوبة على الله للذين يعملون السوء بجهالة ثم يتوبون من قريب فاولئك يتوب الله عليهم وكان الله عليما حكيما
Which translates as: "Forgiveness is only incumbent on Allah toward those who do evil in ignorance (and) then turn quickly (in repentance) to Allah. These are they toward whom Allah relenteth. Allah is ever Knower, Wise."
We have traditions that state "All sins are committed in ignorance" because Allah says: "Those truly fear Allah, among His Servants, who have knowledge" - 35:28. It is none other than our ignorance that leads us to commit sins and if we could really see the ugliness of evil we would never indulge ourselves in it. So the door to repentance is wide open to all to embrace the mercy of Allah no matter what their past holds.
3. You will be reunited with them insha'Allah provided that they are Saved, for Allah says:
Quote
جنات عدن يدخلونها ومن صلح من ابائهم وازواجهم وذرياتهم والملائكة يدخلون عليهم من كل باب
Translation: Gardens of perpetual bliss: they shall enter there, as well as the righteous among their fathers, their spouses, and their offspring: and angels shall enter unto them from every gate (with the salutation):"Peace unto you for that ye persevered in patience! Now how excellent is the final Home!" - 13:23
4. It is an obligation upon parents to teach their children the beliefs and practices of Islam, but if they fail to do so the child continues to be responsible for his own knowledge once he/she reaches puberty (i.e. Bulough). Any obligations that were missed in the past must generally be performed once enlightened to the Truth.
Wasslam