Posting a photo of the note to Twitter, Constable Woods said her niece's note almost had her in tears.
"It's lovely to think that's how she feels about me," she wrote.
The note said Constable Woods was "simply my inspiration!"
"Everything I do that involves hard work and confidence is because of you," the girl wrote.
"Knowing you're such a hardworking, strong, confident, independent woman makes me know I can be too."
The little girl continued, saying her aunt "set goals and reached them".
"And because of that, I'm going to too. Love you aunty Sam."
That note would serve as a source inspiration to Constable Woods when she was diagnosed with early-stage breast cancer.
It came after she complained for months of a dull ache in her left breast every time she wore her police-issued body armour.
Constable Woods put it down to the tight-fitting body armour and even requested permission to order a new one from her sergeant.
Constable Woods said there were no other warning signs that something was wrong with her, and she would not have known there was a problem if it hadn't been for the body armour.
"I've never had any health problems. I've never broken a bone in my body," she explained.
Constable Woods joined the UK Police Force in February last year and wore her body armour daily.
In February of this year, she noticed an aching feeling in her left breast, but the feeling disappeared when she took off her armour after a shift.
"It's hard to explain how it felt. Just an annoying feeling really," she said.
"Not pain. I just assumed that it was just the tightness of it."
Constable Woods said she didn't think it was a health issue because she felt perfectly fine when she wasn't wearing her armour.
"Every now and again I'd get a shooting pain in it too," she recalled.
"I mentioned it to my sergeant and asked how I go about trying to order a new one, as it was hurting."
Constable Woods decided to tell her friends during her birthday getaway in Poland in May.
"All my friends are fellow police officers or prison officers," she explained.
"Two of the group had problems last year with finding a lump in their breast so questioned if I could feel anything."
She told her friends she couldn't, but they told her to go to a doctor for a check-up "just in case".
The pain in her left breast had become quite persistent, so after her return home, she was referred to Pinderfields Hospital, in West Yorkshire.
A consultant there noticed a small indentation on the side of her nipple.
An MRI and several biopsies followed in June, that confirmed the police officer had developed early-stage breast cancer.
Constable Woods revealed her diagnosis to friends and family on Twitter, saying she had been through a "whirlwind couple of weeks, having been told I have breast cancer".
The sudden diagnosis forced the police officer to this week undergo a gruelling three-and-a-half-hour surgery, known as a lumpectomy, to remove the dangerous tumour before it could grow any larger.
Constable Woods, who lives in Yorkshire, will now be forced to wait several weeks to confirm whether the operation was a success.
"I was lucky that I wear body armour, and that gave me the sign," she said.
"I just kept thinking, 'What if I never wore the body armour? How long would it have been before I'd have noticed?' It could have been so much worse."
Constable Woods explained she had wore her body armour almost every day since joining the force last year and was "still getting that same ache".
"But I can't believe that potentially it had saved my life," she said.
If the operation is deemed a success, Constable Woods will undergo a course of daily radiotherapy treatment for up to five weeks.
Constable Woods now wants men and women to check their breasts regularly, explaining the disease can affect people of all ages and genders.
"I just want to share my story because there doesn't have to be any lumps or bumps that you notice," she said.
"I had no visible signs. No warning signs of breast cancer."
Constable Woods says it only takes a few minutes "when you're in the shower or getting dressed".
"Sometimes things are not noticeable, they weren't for me. It can happen to anyone," Constable Woods said.
"At 33 it's not something I expected to hear. But it's caught early which is thanks to my body armour! I would never have known if it wasn't for that."
