Thieves broke into the house in Intake, Doncaster on Thursday, a week after Paris Mulholland's 11th birthday.
In the letter, she tells the burglars "how much they hurt me", that she cries for two hours each night, and they had made her "scared of the dark again".
Paris wrote the letter after speaking to BBC Radio Sheffield.
More on this and other stories from BBC South Yorkshire
Gemma Mulholland said the letter helped her daughter tell the thieves "how she feels, and how it's affected her."
She said: "I really am super proud of her."
Paris' new iPhone, a laptop she got for Christmas, and birthday money was taken from her bedroom, as well as her grandmother's gold watch.
Cash, jewellery and other items were taken in the robbery, along with Paris' and her younger brother Thomas' belongings.
Ms Mulholland said Paris was too upset to sleep in her bedroom after seeing it ransacked.
Lego models were broken and photos taken on Paris' phone during a recent trip to Disneyland may have been lost forever.
Paris wrote: "To my burgler [sic]. I hope you are happy with yourself, I can no longer sleep in my own bed [...]
"Why could you not have left when you saw to [sic] picture of a happy family and when you saw my birthday card, but anyway thank you for scaring me so much I cry 2 hours straight nearly every night."
Ms Mulholland called the thieves "despicable human beings" but said the community had rallied round.
PC Adam Watkinson of South Yorkshire Police said: "Burglary is a highly intrusive crime which can have a huge impact on the victim. In this case, a young girl has been left feeling scared and upset after her home was broken into.
"I would like to reassure Paris and her mum that we are doing everything we can to find the person responsible for this incident.
"It is totally unacceptable that someone should feel scared in their own home and lose valuable possessions."

South Wales Police said 39-year-old Angharad Bullock was reported missing on 28 April, along with the toddler.
Officers are concerned for the welfare of the pair.

The units in North Tyneside and Northumberland have been shut between midnight and 08:00 since December.
Overnight emergencies have been diverted to the recently-opened Northumbria Hospital in Cramlington.
The closures at North Tyneside, Hexham and Wansbeck hospitals, will continue for a further three months.
The Northumbria Healthcare NHS Foundation Trust said the impact on patients had been minimal as the units were underused at night.
Staff from the units have been moved to the Northumbria Specialist Emergency Care Hospital, in Cramlington, which opened in 2015, to deal with a "very high demand" in day-time admissions.
A spokesman said it would be "inappropriate" to deploy staff back to the urgent care centres overnight at the current time.
He added: "This is to ensure best use of precious staffing resources to deliver patient care where it is needed most."
The trust said since opening the Northumbria hospital in June 2015, activity overnight at the urgent care centres had been "minimal" with, on average, less than 10 overnight attendances a day across all three centres.
The spokesman added: "We'd like to assure residents that this decision to temporarily extend the current arrangements at our urgent care centres has not been taken lightly.
"It was entirely correct for us to put in place the interim changes in the wake of services being extremely busy across the NHS. They have helped us successfully meet the high demand we continue to see, even now during the summer.
"As a result, it would simply not be the best use of our staff's time and expertise to deploy them back now."
An 11-year-old girl has written a letter to burglars who raided her home and stole birthday and Christmas presents.
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Police are searching for a woman who has gone missing with her three-year-old son.
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Three emergency care units are to remain closed to overnight patients in an effort to keep "precious staffing resources" focused on daytime care.