Now for the translation...

小時候，When I was small, [Your version is also good.  Great job!]

幸福很一件很簡單事；happiness was a very simple thing.

如今，Now, 

簡單是件很幸福的事。Simplicity is happiness. [You are doing very well in translating from one mode to another to express your thoughts]

我們很在乎得失，We care only for our gains and losses.

在乎別人對自己的印象。We care how others perceive us.

生活的壓力讓我們沒有一點幸福感。Pressures in life does not allow us to feel an iota of happiness. [This is a more literal translation.  However, what you wrote is also great.  The meaning is the same; just different in expressing]

在重慶的一段時間，When I was in Chungking, [Finally I understood what the place was]

我突然又品嘗到幸福的滋味。I suddenly tasted happiness.

雖然工作累，Although work is tiring, [your use of arduous is good]

但我過得開心。I was happy.

為了每天的工作目標，For reaching our daily work goals,

我們一起討論方案，we would sit around the table to discuss and

驗證我們的方案。to test our hypotheses.

我們的努力換了周圍人的好評，Our effort paid off and earned constructive criticism from all around us.

和心與心的交流。From this, we gained great camaraderie.

在深圳時，When we were at Shenzhen,

我們可能不是最好的兄弟，we weren't exactly the best of brothers. [Now I understand what you are talking about.  This is how this is translated. Now you know how to express it in English].

當我到了重慶，However, when we reached Chungking,

他們是我最熟悉的人。we were no longer strangers. [This is better than translating directly as, "we became very familiar with each other" or "we know each other very well".]

飯桌上，At dinner, [Your version works very well, it gives the local flavour of the setting]

我們談論著在深圳的快樂時光。We reminisced/talked/chatted about the good old times in Shenzhen.

不知不覺一打啤酒就喝完了，Before long, a dozen of bottles of beer were gone. [done is the more correct translation but shows that you are not very colloquial in English.  As translation, there are many factors to choose which is the best word to use.  To express better and have a better flow, 

it is not always best to have a literal translation]

回到賓館，Upon reaching the hotel,

我們躺在床上，we were on the bed,

還不停的談論著。still continuing with our  tête-à-tête/gabfest [These words are better than just "talk", "discussion", "dialog", "exchange".  They sound too serious.]

逐漸的就有人睡著了，Gradually, someone fell asleep,

最後全睡著了。the rest soon followed suit.

我們忘記所有的不快樂，We had forgotten all the unpleasantness;

忘了生活的壓力。so too were all life's pressures. [the literal translation "forgetting all life's pressures" sound too heavy for my taste. This separates the men from the boys. ]

簡單的生活模式就是我的幸福。Simplicity in life is my happiness.

As you can see, although the meaning in both versions are the same and people can understand, the tone and fluidity comes out a bit different.

Correcting from your English only.

My Happiness

In my childhood, happiness is was a very simple thing. Now, simplicity is happiness a happy thing. [Not wrong but my preference] We all care about our gains and losses; and care about the we want to impress of others to us. Life Pressures in life makes us couldn’t feel any take/suck away all [our] happiness. The time when I was in CQ [Give the full name of the place.  This is laziness.  Remember, your written essays are meant to be read by people all over the world.  Perhaps the locals know what this means but others don't.  All writings should be written with clarity in mind], I suddenly tasted the taste of happiness. Although the work was so arduous, my life was full of happiness. For our To attain the daily work’s goal, we sat together to discuss the proposal and then to test our hypotheses it. Our effort won the others praise from others by this intimate setting. and heart to heart communication. When we were in SZ, maybe we were not the best fellow, but we were in CQ, they were the most familiar person??? [Great that you have provided your Chinese version.  I shall go there to see what's happening and translate]. Around the dinner table, we all talking ed about the happy times in SZ and a dozen bottles of beer later, had been finished everyone was in a jolly mood.imperceptibly. Returning to the hotel, we lied laid on the bed talking about it to continue ally our camaraderie. However, gradually, someone fell asleep and soon all of us fell followed suit. asleep eventually. We forgot all the unhappiness and life's pressures and its associated unpleasantness. The simple way of life style is my happiness.






小時候，幸福很一件很簡單事；如今，簡單是件很幸福的事。我們很在乎得失，在乎別人對自己的印象。生活的壓力讓我們沒有一點幸福感。在重慶的一段時間，我突然又品嘗到幸福的滋味。雖然工作累，但我過得開心。為了每天的工作目標，我們一起討論方案，驗證我們的方案。我們的努力換了周圍人的好評，和心與心的交流。在深圳時，我們可能不是最好的兄弟，當我到了重慶，他們是我最熟悉的人。飯桌上，我們談論著在深圳的快樂時光。不知不覺一打啤酒就喝完了，回到賓館，我們躺在床上，還不停的談論著。逐漸的就有人睡著了，最後全睡著了。我們忘記所有的不快樂，忘了生活的壓力。簡單的生活模式就是我的幸福。