( I like how two good examples, OLE and carnival, are connected. Even managed to squeeze in math olympiad at the end!
However, the jumping back and forth chronologically is distracting. Why not start with OLE?
I suggest removing some unnecessary words, to clear space for more specific examples.)

Through the Open Little Eyes conference I attended in April this year, I came to understand many global problems in  child welfare. (Don't use the word 'issues'; it's vague to the point of saying nothing. Also, give a few examples of these global problems; expect that the reader doesn't know anything.) I was one of two primary six students who represented my school, and there I experienced a simulation of inequality and exploitation. (simulation? vague, describe a little) The vulnerability I felt made my tears well up in frustration. The feelings of helplessness towards the blatant discrimination that I felt was unforgettable. (sentence is awkward, try: While struggling against acts of blatant discrimination, I will never forget my feelings of helplessness and frustration. )


The conference fuelled my desire to do something special for children who struggle daily against bleak conditions. (saying you're middle-class doesn't say anything important - omit)  Fortuitously, Singapore Chinese Girls’ School organised a fund-raising carnival  in that same month, and many underprivileged children from organizations (such as? Give one/two examples. Specificity is good) have been invited.  ( Don’t waste words saying unimportant things like: …fund school programmes.  I think it’s not important for the essay theme, and neither do you, so don’t bother write it. At the very least, don't say 'More importantly...' because you're not respecting your previous sentence.)  (Too many repetitions like: I was excited to contribute to this event. You imply this in your next sentence.)  I shared my excitement with my three best friends, and we decided to set up an unprecedented horse-riding booth. Our team penned a project proposal, sealed it in an envelope of hope,  and delivered it to our principal. (Notice the previous sentence flows better than what it was) I could not believe my ears when she approved our proposal! (cut unnecessary words, like in previous sentence) The next step was to raise $2,500 – the cost of enlisting Horsecity to provide a booth. (cut out words, as in previous sentence)  Over the next month (don't say next one month, say next month), my friends and I corresponded with Horsecity and pleaded for donations. I often felt demoralised because I could not convince some friends and parents to support the cause. I began to wonder if I had been overambitious (having doubts is implied in previous line, don't repeat yourself) - all the other student-initiated projects were regular small-scale stall activities. However, giving up meant that the underprivileged children may never get another chance to ride a horse - I could not accept this. (Much shorter now)
(sat my friends down is a waste of words) I encouraged my friends to brainstorm for other fund-raising ideas. We raised $700 by organising a flea market, where we sold our classmates’ old toys and books. (I thought of? but that's too obvious, of course one has to think about it, so it says nothing) We sold the idea to potential donors by emphasizing the humanitarian aspect of our project. (There seems to be a sudden gap here of $1800, which any reader would like to know where it came from. Perhaps expand, it it is interesting) Eventually, we succeeded in raising $2,500 in time for the carnival. (project began full swing? these words say nothing.)
On carnival-day, there was much excitement surrounding our booth, and the crowd proved overwhelming. Responding to the pressure, I quickly enlisted additional friends to help the enthusiastic participants as they... (what do they do exactly? be specific) . Unfortunately, I was called away by teachers to participate in the Mathematics Olympiad that same day, and I could only listen to my friends’ happy accounts after the carnival. I was very proud to hear that the underprivileged children had a fantastic time.  (perhaps there is an example of how much a underprivileged child appreciated the ride? something to show instead of tell.)