关键是“孩子的最好权益”best interests of the child.

无法预测胜算，看各人的情况，关键是小孩多大。外婆岁数大是个致命缺点。小孩自己的愿望也很重要。

孩子的权益因素英国法律规定如下
http://www.legislation.gov.uk/ukpga/1989/41/section/1

UK Children Act of 1989 defines:
1 Welfare of the child.

(3)In the circumstances mentioned in subsection (4), a court shall have regard in particular to—

(a)the ascertainable wishes and feelings of the child concerned (considered in the light of his age and understanding);

(b)his physical, emotional and educational needs;

(c)the likely effect on him of any change in his circumstances;

(d)his age, sex, background and any characteristics of his which the court considers relevant;

(e)any harm which he has suffered or is at risk of suffering;

(f)how capable each of his parents, and any other person in relation to whom the court considers the question to be relevant, is of meeting his needs;

(g)the range of powers available to the court under this Act in the proceedings in question.