When a 9-year-old girl offered me sex in return for flip flops, I knew life would never be the same.

Yes, sadly, you read that right. She made the offer when I innocently bought her some flip flops - and that was the moment my life changed.

Dreaming of becoming a lawyer and researching universities, I set out on a church mission trip to war-torn Sierra Leone. The poverty was horrific and the brutalities were extreme. This country had been ripped apart through a civil war.

I'd previously travelled across Africa and also spent a month in Colombia, but nothing could prepare me for this.

Tall for her age, though desperately thin, Felicity was just 9. She'd noticed me talking to 6-year-old Solomon, another street kid in the city of Bo, and she'd come over to help.

I didn't know it just then, but my life was about to be turned upside down.

Solomon spoke no English but, with Felicity's help, I got to understand how both of them were on the streets. They were begging. They were well known in this dusty square, with its wooden bench stalls and its piles of second-hand clothes for sale. Solomon was begging on behalf of his grandparents, because he could get more money than they could. Felicity seemed to be on her own, and never mentioned any family.

My friend Amanda and I helped them both. We bought some clothes from a market stall to replace the rags that Solomon was wearing. And for Felicity I bought some pink flip-flops.

I'd noticed straight away that Felicity was barefooted, and it seemed a small gesture to get her the shoes. She chose the pink ones which cost less than £1.

I invited the two of them to join us later that day when we were to drive out to an open-air church service. As I stepped out of the hotel in the early evening, the sun was going down and the dust from the streets was beginning to settle as the traffic lessened. I saw two children running towards me - Felicity and Solomon. They sprinted over and gave me the biggest of hugs. I was delighted they had remembered, and pointed them to the vehicle we were travelling in.

"But don't you want me to go to your room?" Felicity asked. I didn't understand.

"No. We're going right now. There's room in the vehicle – in you get!"

"Shouldn't I wait in your hotel room?" Felicity repeated.

It was then that it hit me. I was a woman in my 20s at the time and, horrifyingly, the little girl for whom I'd bought these flip-flops thought that I was doing it in return for sexual favours.

My mind was racing. How could she think that? What terrible things had happened in her 9-year-old life for her to even go there?

I looked at her. And I was angry. Angry with all the poverty and corruption. Angry with the abusive men and women who could do that to a child. Angry that Felicity had been robbed of her childhood.

And that's when I knew. The promises God had given me years before regarding working with children in poverty were no longer just promises. It was time.

Felicity's face became my personal non-negotiable moment with God. He had called me. Standing in front of me was the very reason for the call. I knew what I had to do. I looked into her eyes and knew that God was setting something beautiful in motion.

I'm often asked what happened to Felicity. Sadly, I don't know. Back then there was no children's home, no website, no TV interviews, no books and no money. It was just me. I couldn't rescue her, I couldn't bring her into our family and I couldn't even give her any money. I had nothing.

Is Felicity still alive today? Was she further abused? Heartbreakingly, I just don't know.

But what I do know is that she changed my life. Fast forward over a decade and One By One – the charity I founded in 2012 – is now reaching more than 10,000 children per week. We have established our HQ in Kenya and more recently started rescuing children in Pakistan, where we have built a safe house and seen 50 children freed from slavery.

Every time I welcome a new girl I think of Felicity. Earlier this year we had a 10-year-old girl in our Sunday School programme in Pakistan who was raped and left for dead. Miraculously she survived. Another girl, Mercy, who was just 3, sadly didn't and died shortly after the attack.

These incidents certainly don't get easier. Once you see something, you can never unsee it. But it leaves us with a choice. We either walk away or we keep fighting for justice.

Felicity gave me my life-defining moment. She doesn't know that she was the catalyst for tens of thousands of lives that are now being impacted. When we are faithful with the little, God will trust us with more.

I'm often asked where the name One By One came from. Well, now you know. And that will be my life mission – to help girls like Felicity, one at a time.

Becky Murray is the founder of One By One and the author of Embrace The Journey. You can find out more at www.onebyone.net