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Fill light will lift the shadow detail and reduce the dynamic range of the scene (the difference between the relevant highlight and relevant shadows) to within the camera sensor's dynamic range, thus preserving both highlight detail and shadow detail.
However, the other problem with shooting in these conditions is that the sun is a very hard light source, creating sharply defined shadows (as in the shot of our toddler building sand castles). To address that issue, I like to use the sun as rim light instead of the key light. With the subject's face in shadow, there's no harsh light on the face. But that leads to the previous problem of dealing with the exposure of the subject's face which is now hidden in the shadow.
We could use a popup flash but that is a hard light source just like the sun, so we would be back at square one. It's possible to use an umbrella or softbox but the beach is usually very windy, making it hard to handle a suitably large umbrella or softbox.
An easy alternative is to use a ringlight (aka ring flash), which produces light that appears almost shadowless (as long as the subject is not next to a wall). A ringlight produces flat, even lighting that doesn't help reveal the subject's three-dimensional form - but that's ok under these circumstances because that's what we would expect a shaded subject to look like, thus making the use of the flash less obvious. The ringlight's reflection will show up on sunglasses and other reflective objects but it's easy to clone out if you want to. Here I left the reflections alone.
Coco Ring Flash
Use Flash in Daylight?
Fill Flash: Summer Pool Party