is that correct in grammer?

it's good enough but there are some little mistake...

FIRST OF ALL, I would like to talk about the advantage OF the logistic part. The advantage of the event WAS good AT handling timing. When the participants were playing, some TEAM mates HAD TO prepare FOR the next game. For example, the participants went out to play ON the next game after their tea time. At the same time, some TEAM mates cleaned the classroom and took the giftS out. Then, our group had enough gifts to all participants. They FELT HAPPY IN THE END (OF THE EVENT).

雖然我d英文一般，但係都讓我試試 rephase一下d 英文啦

First of all, I would like to talk about the upside effect in relation of logistics. We focused on the time management as being a significant part of the plan. When some of the group members were occupied with coordinating a game for the participants, the rest of the group prepared for the settings of the next game. For an example, while the participants were out playing after the tea time, the unengaging group members made use of the time cleaning up the classroom and laying the presents out ready for a surprise. At the end of the event, finding out there were enough presents for each of them, the participants left with joys and a sense of accomplishment.

註：
一開始，我覺得都係用upside effect好過advantage，”好影響”好似比”好處”較貼切
time management 好聽過就咁handling timing
第三句，你首先話participants... 跟住話group (members)... 咁樣我諗冇咁好，因為你用緊participants同group member黎做對映，你應該以搞緊活動既group members同準備緊既group members黎對映，所以我將前半句既subject做左group members，而coordinating a game for the participants做object
尾二果句，你用then黎開頭，咁係ｏｋ，但係你之前先用過個For example黎舉例，咁你一直都係講緊同個example有關既野，你唔可以就咁用then去突然跳左去另一個point，講其他野咁，所以我用At the end of the event咁樣去間斷個point

有野想向樓上kellshine既回覆請教，得罪了：
Play (a game) 其實都ｏｋ，但係你改用左play on (a game)，這樣意思上不是有改變嗎？是不是有甚麼特別理由？請指教