Aquí tienes lo que está mal -igual se me ha escapado algo, pero nada importante-; no lo he corregido directamente porque va en contra de mis principios. Debajo tienes el código que suelo usar en las correcciones, pero si tienes alguna duda no dudes en preguntar. Hay un this que podría ser that, pero yo lo veo bien, así que no lo he corregido. Imagino que os escondisteis al lado de una puerta; si no es así, tendrías que usar behind. Tampoco pondría el número de atracadores en número, sino en letra, y ese [who?] que he puesto es porque parece que los arrestados fueran los policías 

.
Aparte de eso, algún fallito en los verbos, lo de "in a quick way" que queda muy cutre y lo cambiaría por un adverbio... poca cosa más. En general está bastante bien; no son fallos gordos. 
When 
i [P] was eleven [P] one day we went to the mall to have a nice day shopping and having fun.
Then something strange happened. 
Hotters [WW] started to sound and people started to run. I was so astonished since I 
have [T] never 
before [WO] 
heared [irr] that sound before in that mall.
My parents told me to run and hide because this could be a robbery or even worse: terrorism.
We hid by [*Prep?] a door near the fire exit and we watched the scene: 3 guys who 
stealed [irr, T] lots of jewels and money. 
Then the police broke 
in [Prep] the mall just like a 
breathe [WW] and in 
a quick way they [who?] got arrested since their weapons were fake.
All in all [P] that was one of the best personal 
expierences [SP] in my life.
T = tense 
Irr = irregular verb
P = punctuation 
WO = word order 
Prep = preposition 
WW = wrong word 
GR = grammar 
SP = spelling