What exactly is that evidence that 10 year olds kids understand to be fact?
Well that's easy.
I would ask them to bring a few family photos, and say;
Well kids, evolution just means change over time - that the offspring of any species is a little different to the parent, and those little differences add up to bigger differences over many generations.
Look at a photo of your father, notice how he looks similar to you - but is a little different? Now look at a photo of your grandfather and see how it is also similar but a little different to you.
THAT children is evolution. Those small changes you can see between you and your dad in the photos add up to larger changes over many generations.
Now modern scientists can look at the genetic code of a species and compare it to that of the species offspring - the differences are proof of evolution. In biology evolution has a more specific definition - changes in allele frequency over time. Now all that means is that small changes in a species genetic code in every generation add up to larger changes over many generations.
So as you can see kids, evolution is just change over time - and that species change a little over time can be (and of course has been) observed, measured and hence proven to be a fact.