兒子 = son 孩子 = child, kid, brat (孩子 sometimes may also be applied to adults younger of your age or to ridicule others when having quarrel. 小孩 = child Example: 這是他們的兒子。This is their son. 孩子你還是快回去吧，別出來丟人現眼。Act your age, kid, and get out of here now. 五歲以下小孩免費使用。Free for children under age of 5.

儿子refers to son, so male offspring specifically, whereas 孩子and 小孩Are similar in meaning as they both mean child but 小孩 is more of an informal way of referring to a child in comparison to 孩子.

儿子=son 小孩，孩子=child 孩子 is more formal than 小孩, and 孩子 sounds warmer and more polite. The two words are interchangeable, but I prefer to use 孩子.

Like the English word "child", 孩子 can mean either "young child" or "son/daughter", depending on its context. e.g. 男人,女人和孩子 = men, women, and children 他有3个孩子, 都结婚了. = He has 3 children, all married.

儿子 = son 孩子 =小孩 = child