The mother made a promise that pleased all her children

同位语从句与定语从句有一个最大的区别就是：同位语从句是有自己的主语的,记住这一点就不会再混淆了.例如：
The mother made a promise that she would buy candies for her children.此处that引导的就是一个同位语从句,从句的主语是she.
而：
The mother made a promise that pleased all her children中that引导的定语从句的主语是先行词a promise.
另外,从意思上也可以区分的：
The mother made a promise that she would buy candies for her children.
妈妈承诺她会买糖果给孩子们.
这儿“她会买糖果给孩子们”就是“承诺”的具体内容,也就是说,a promise就是she would buy candies for her children,所以是同位语从句.
The mother made a promise that pleased all her children.
妈妈做了一个让所有孩子们都很开心的承诺.
这儿“让所有孩子们都很开心”是用来修饰“承诺”的,所以是定语从句.

That指代前面的那件事,这个一种修饰关系

同位语从句两者是可以相互代替的，它们表达的是一个意思。定语从句则是修饰关系。