From what I understand a child's request can be considered a change in circumstance. However, the decision is not based on that particular request. The decision is still based on what is in the best interest of the child.
In my opinion the BM would be a fool to submit to the courts a letter from a child saying "I want to live with my mom because she buy's me stuff."
In my opinion the BM would be a fool to submit to the courts a letter from a child saying "I want to live with my mom because she buy's me stuff."