erhua/erization is a kind of pronounce habit of north Chinese people, especially in Beijing. Most of the time, it means a casural and relax emotion of the speaker. However, as the habit become so strong, that some words with "儿” has special meaning, and result in new words, ex.: 头---head 头儿--leader；（开）会---meeting （一）会儿---a while; 眼---eye 眼儿---small hole There is no rule for this, so just learn when meet them.

Erization is a kind of pronounce habit of north Chinese people.多数时候是中国北方人的说话习惯，不需要考虑的很清楚。但是有一些就是那个用法，就像“女儿”。 I hope you could understand.

“孩儿、女儿 ”both of them just a a kind called,another one "哪儿“、”哪“、”事“、”事儿“,they are the same means,doesn't have anydifference..

It seems to me '儿' used thus as a word final sound is really not different to some of the sentence final particles endings la 啦, ne 呢, ya 呀... and such speech behaviour is common to many languages; it's function is usually to convey some tone or mood ... what do you think? here is an extract from Wikipedia Sentence-final particles are common in the Chinese languages, including particles such as Mandarin le 了, ne 呢, ba 吧, ou 哦, a 啊, la 啦, ya 呀, and ma 嗎/吗, and Cantonese lo 囉 and ge 嘅. These particles act as qualifiers of the clause or sentence they end. Sentence-final particles are also present in Japanese[3] and many East Asian languages, such as Thai, and especially in languages that have undergone heavy Sino-Tibetan influence, such as the Monguor languages.

兒化是中文標準運動的時候從北京方言加進來的東西 很多地方基本上不會用到兒化 在台灣，太頻繁使用兒化很容易讓人感到詭異與反感。