Actually both are crucial factors but i can say that definitely it is environment that effects most. A child has to adjust with the situation and challenges he is getting exposed to and gradually he accepts the situation and moves on. It sometimes gives him positive shade or sometimes negative but he grows with environment.
You ask the following: Heredity and environment, which of them affects child development most?
As I see it, your question is a different way of framing the following question: nature and nurture, which of them affects the child's development most?
Let me say that answers to this question passed through three phases.
1) The how much phase. In this phase, scientists tried to quantify in terms of percentage how much heredity and enviroment affect the child's development. There is now a widespread consensus that it is not possible to quantify the influence of heredity and environment in the child's psychological development. Because of this, it appeared a second phase.
2) The which phase. In this phase, scientists tried to identify which factor (heredity or environment) affects the child's development most. The question you ask belongs to this phase. There is also a widespread consensus among researchers that it does not make much sense to ask such a question, mainly because there are occasions (e.g., to behave while attending a funeral) on which environment seems to be more influential than heredity in shaping the child's development and behavior, and other occasions (e.g., to perform in a mental test) on which heredity seems to be more influential than environment in shaping the child's development and behavior.
1) The how phase. In this phase, which is the current phase, researchers try to examine how heredity and environment interact to bring about the child's development in general or one of his/her behaviors in particular.
In a nutshell, the idea is not to see the extent to which, in terms of percentage, heredity and environment affect the child's development (phase 1) nor is it to see which factor, heredity vs. enviroment, affects the child's development most (phase 2), but how heredity and environment interact in shaping the child's development. So, the question is not an either/or question, but an interaction question. That is, the child's development in to be seen as a result of a dynamic interaction between the child's biological heritage and the physical and social milieu in which s/he grows up.
I hope that I has got your post and that this helps.