She is very protective of the children in her custody and can get very aggressive to protect them from strangers or dogs.

protective 是形容词，由动词 protect（保护）加上形容词后缀 -ive 构成，表示"保护的、防护的"。这里描述她对孩子们有强烈的保护意识。 protective of 是一个固定搭配，表示"对...有保护欲的"，强调对某人或某物的强烈保护倾向。