I've done both with my son (when he was 6). I agree with Jiejie and just try to tackle Xian.

Xian was fun for a short day trip. We had 2 nights which was plenty. My son had read up on the Terracotta warriors so he enjoyed the half day trip (we had our hotel arrange a driver).

For the old city, we rode bicycles on the city wall. Did a tour of the Bell Tower and saw the Big Goose pagoda. For some reason we ended up at Pizza Hut that night to the delight of my son.

Yangshuo was a longer stay as we met up with friends to hike, bike, kayak and rock climb (without son). Much more of the laid back part of our trip through China. However, with only 2 days, you will feel rushed.