Since he left the family, the kids only see him as their former father.

former 是形容词，意思是"以前的、从前的"，表示某人或某物过去的状态或身份。 father 的意思是"父亲"，指有血缘关系或法律上承认的父亲身份。 former father 指"曾经的父亲"，强调这个人过去是父亲但现在已不再履行父亲职责或失去父亲身份。