A five-year-old American girl on a mission is fast becoming an internet sensation after she travelled from Los Angeles to Washington DC to break ranks and hand Pope Francis a poignant message.

As the papal procession inched its way through Washington's National Mall on 23 September, little Sofia Cruz stepped out from the crowd and boldly made her way towards the Popemobile clutching a yellow T-shirt. She was quickly stopped and ushered along the sidelines of the barriers by a security guard, but she had caught the attention of the "people's Pope", who encouraged her to come forward.

Another guard scooped her into his arms and carried Sofia towards the vehicle to meet Francis, the self-proclaimed "son of an immigrant family" and the two shared an emotional embrace cheered by thousands.

But her mission would only be complete when she handed the Catholic leader a note seeking his support to push for immigration reform. Like five million children living in the US, Sofia fears that she could be separated from her parents due to their differing immigration statuses. Her parents hail from Oaxaca, Mexico and are considered "illegal" immigrants while she is a US citizen.

About 11 million people – some of whom have native-born children – live illegally in the US. Reciting the contents of her letter to the Guardian, Sofia said: "I want to tell you that my heart is sad. All immigrants just like my dad help feed this country. They deserve to live with dignity. They deserve to live with respect. They deserve an immigration reform."

Sofia had travelled to the capital along with a dozen members from her Nuestra Senora Reina de Los Angeles parish, who said: "She handed the pope a letter asking him to support the drive to legalise undocumented migrants living in the United States."

In her note to the Pope, Sofia also included a drawing with a message in Spanish that read: "My friends and I love each other no matter our skin colour." Today (24 September), Sofia's message – representative of millions of undocumented people – was communicated by the Pope in an historic speech delivered to Congress.

On immigration, the Pope said:

"In recent centuries, millions of people came to this land to pursue their dream of building a future in freedom. We, the people of this continent, are not fearful of foreigners, because most of us were once foreigners. I say this to you as the son of immigrants, knowing that so many of you are also descended from immigrants.

"Tragically, the rights of those who were here long before us were not always respected. For those peoples and their nations, from the heart of American democracy, I wish to reaffirm my highest esteem and appreciation. Those first contacts were often turbulent and violent, but it is difficult to judge the past by the criteria of the present. Nonetheless, when the stranger in our midst appeals to us, we must not repeat the sins and the errors of the past.

"We must resolve now to live as nobly and as justly as possible, as we educate new generations not to turn their back on our 'neighbours' and everything around us. Building a nation calls us to recognize that we must constantly relate to others, rejecting a mindset of hostility in order to adopt one of reciprocal subsidiarity, in a constant effort to do our best. I am confident that we can do this."