What should children do if their parent die in debt and children don't know about it, even the person who gave the loan not reminding or even mentioning about it. What children should do to make sure that his/her parent will remain save from Hellfire if they forget to inform about it.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

If it is confirmed for the children that their parents had debts, then the matter is one of the following two cases:

1-     If the parents had left any wealth, in this case debts should be paid before dividing the inheritance.

2-     If the parents had not left any wealth, in this case the heirs are not obliged to pay the debt, but if one of them does so, he will be rewarded, as this is being kind and dutiful to the parents. Nonetheless, if the parents wanted to repay their debts but they were not able to do so, then they will not be accountable for it, as the Prophet said: "Whoever takes people's money while wanting to repay, Allaah would help him to repay, and whoever takes people's money while wanting to spoil them, then Allaah will spoil him (i.e. spoil his wealth and punish him in the hereafter)."  [Al-Bukhari] Of course, in this second case, the silence of the creditor is not enough to neglect his right if the heirs know about it, as the silence might be because of shyness and the like, so one should give him back his right unless he renounces it with his own will. 

Allaah Knows best.