What are the methodological problems in attempting to discover how children think and feel

What are the methodological problems in attempting to discover how children think and feel?

Answers given by one of my classes:

Children tend to give their parents opinions rather than the own
Children may not understand
Children might treat things as a game
Children have short attention spans
Children may need an incentive
Adults find it difficult to relate to children
The permission of parents must be sought
Children cannot express themselves very well
Children feel threatened
Children's emotions are different from adults
Children need to be relaxed
Children try to please the experimenter by giving the answers they feel ought to be given
Children do not always appreciate the difference between reality and make believe
Children are easily influenced