This is not so easy, as I am sure you appreciate, but maybe start with explaining the sizes of things - this may help https://scaleofuniverse.com/en
Then she can see that there is a limit to how large the universe is and it is defined by how far we can see, we can see nothing beyond this with our technology. But it could be considered like very very big balloon which is getting larger & larger, and we are one tiny dot inside this balloon. This is where that scale animation helps.
An astronomer with skills in education of young people probably would be able to explain it better