1. Uneven time distribution: As the number of children increases, parents should spend more time taking care of their children. However, because the two children may be different in age, personality and hobbies, parents need to provide different attention and care for each child, which makes it difficult for parents to allocate time and energy equally among their children.

2. Uneven distribution of resources: Parents also need to provide their children with the resources they need for life, including food, clothing, housing and transportation, education and so on. But the allocation of these resources may be affected by many factors, such as children's age, gender, academic performance, hobbies and so on. Parents may have to choose between their children, which also leads to uneven distribution of resources.

3. Parents' personal preferences and prejudices: Parents may be influenced by their own personal preferences and prejudices when dealing with their children's problems and decisions. For example, they may be more inclined to help more naughty children and ignore those who behave better but are quieter. This may also lead to the problem of fairness.

4. Different family environment and background: Different family environment and background may also make it difficult for parents to provide equal opportunities and support for their children. For example, the differences in family economic conditions, the health status of family members and the stability of family relations may all affect parents' decision-making and resource allocation.

Therefore, it is difficult for parents of two-child families to be completely fair, but they can try their best to balance the needs and resource allocation between children in different aspects to ensure that children receive proper attention and support.