请教这里为什么只能用Since，不能用When？

如果用when, when she was a child, 说的是过去时间，她小时候

而这里用since she was a child，与后面用完成时态呼应，即：
自从她小时候以来，就一直住在上海

例句：
When she was child, she lived in Shanghai.
她小时候住在上海。（言外之意：她可能后来不在上海住了。）
Since she was a child, she has lived in Shanghai.
自从小她就一直住在上海。（言外之意：她至说话时仍住在上海。）

主要是因为后半句是现在完成时
现在完成时可以用来表示发生在过去某一时刻的,持续到现在的情况

这里说明,她依然住在旧金山

如果是when 只能表明过去的某个时段

(Since) she was a child,she has lived in San Francisco.
这样更好懂些：

she has lived in San Francisco，(Since) she was a child

她一直住在旧金山， （自）她是一个孩子起。

从……时起。

When 呢？是当……时。

很明显的差别。

当她是小孩子时，她住在旧金山。

When she was a child , she lived in San Francisco.
当那时，对现在没有影响，不是完成时，since自从，自然有完成的意思，对以后有影响的意思。

2.是不是有Since的句子，从句(she has lived in San Francisco)都要用现在完成时？

也不是,如：
Jim has been living in Beijing since he came here （3 years ago）.

上面括号里是此前提的注释：(Jim came to China