The service for Freya Bevan, who died last month, took place at Margam Crematorium, Neath Port Talbot, with people paying tribute along the route.
Freya, from Neath, had been undergoing treatment in the US after it returned, prompting a £200,000 fundraising drive last year.
Her family said heaven had gained the “sweetest and most beautiful warrior”.
In a Facebook statement, they asked people to wear pink in her memory.
Freya was first diagnosed with a brain tumour in May 2014, at 22 months old.
She underwent two major brain surgery operations, chemotherapy and a stem cell transplant.
And in February 2015, she travelled with her family for the first time to Oklahoma for proton beam therapy.
They returned to Wales in May that year but in October 2020 a routine scan showed a tumour had returned.
Freya had to then undergo a nine-hour operation to remove it and her family decided they would return to the US for more proton beam therapy.
However, she died on 27 December.
“No more pokes, or pills or hospital visits, you are free now our beautiful angel,” her family said in a tribute last month.
“Our hearts are completely broken, as we share the news with you, that heaven has gained the most sweetest and most beautiful warrior there ever was.
“Our brave, beautiful, precious Freya is now healed from cancer and getting big cwtches in heaven from her beloved great grandmother.”
In a tweet on Saturday, Swansea Rugby Club said: “On this extremely difficult day, the boys will wearing pink bows on their jerseys in celebration and as a tribute to Freya.”