But the length of the real proof is beside the point. We teach them about 1 and 2 before going to 1+1=2. And when we do go to 1+1=2, we show it to them using objects and counting. We don't simply tell "here's the function for addition, just put numbers into to to solve problems".
Then addition is "proved" in terms of those.
Then multiplication is "proved" using addition.
One building on the next.