correction

首先你要明白，作為名詞的mainland必須有冠詞the，而且介系詞是用on
所以，全句應為：
Why don't (you) let them stay longer with their family on the mainland?
如果，你想說中國大陸，則要說mainland china，這個複合名詞第一個mainland是作形容詞用，所以，就看如China一樣，而介系詞用：in mainland China
所以，全句可以說：
Why don't (you) let them stay longer with their family in mainland China?
詳情可參考【征服英語】：http://education.atnext.com/index.cfm?fuseaction=Article.View&articleID=6860767&issueID=20070301

Why don't you let them stay with their relatives in Mainland for a longer while?