What comes to my mind is, hao3 Wei4 kou3 好谓口(bon appetite)，chi1 duo1 dian3. 吃多点(Eat more). But there are too many dialects. So there are too many expressions.normally I hear qi3 kuai4,起筷(子)。(move chopstick)or chi1 fan4x x x吃饭(eating). i come from South of China.so I think we don't say common phrase.

usually we say nothing. But if you visit your friends or any other home . They 'll say something like"粗茶淡饭，不要嫌弃" "请随意，不要客气""吃好喝好"etc.

I think the Taiwanese have something like 开动咯！(which I believe is influenced by Japanese, due to the long Japanese occupation over Taiwan) However, for us Mainland Chinese, it would simply be “赶紧吃吧”，“菜都凉了，还等什么？,which literally means”Quickly eat!" "what are you waiting for? The food is getting cold!"

真的吗？没有"itadakimasu"吗? 我感激你直率地说。但是，现在我要去哭。。。

No, there is not. You just start eating. As simple as that.