This is all an arbitrary talk since we have no basis to conclusively know how deeds are going to be valued on the day of Judgement. The one whose parents did not make it to Jannah, probably had to go through hardships in life that other children did not have to go through who themselves along with their parents made it to Jannah. Those hardships alone could land the former category into the same level as the latter category.
There are so many variables surrounding a human’s life, human’s intent, human’s actions and their surroundings that only ‘Being of God’ can fairly and justly judge a human, while at the same time giving regard to each and every minor and intricate factor affecting a human’s life. The former category could land in higher levels because of hardships, and the latter category once becoming eligible for Jannah could land in further higher levels because of their parents (it could that the prayers of those parents would have some role to play in equating the level of children with that of their parents and vice versa). The way we all Muslims pray for Prophet Muhammad (sws), those prayers could be used to raise our beloved Prophet (sws) to the highest levels of Jannah. This does not mean that God needs our prayers to do that, but it would truly be a great source of honour and happiness for both the parties. Based on the same principle, one can hope that Prayers by Person ‘B’ in favour of ‘Person A’ could help Person ‘A’ in raising of his levels or both of their levels, provided once they become eligible for Jannah. There is no unfairness in it, since prayers were there to begin with.
The possibilities are endless and only God has the capacity and capability to truly judge His creation on the day of judgement. Nothing can be said conclusively based on our limited knowledge.
ALLAH knows the best.