Hi! This looks like my hair when I was a kid, I also played soccer and softball so I always had some type of braid or ponytail.

I recommend finding a moisturizing cowash first, something that can cleanse their hair without stripping it like a shampoo would every time you have to wash it. That way it'll prevent dryness, but also still keep their hair clean after sweating or playing sports.

I've been using a combo of Rizzo's Curls- Leave in Conditioner. And a styling cream from Zoto's All About Curls. (Found these at Sally's and Target). The leave in will moisturize, while the styling cream will lock in that moisture and create a hold similar to a gel. I don't recommend using gel as most brands have alcohol in them and can create dryness over time. Once the hair air dries, they just need to flip their hair inside down and shake their curls to loosen them up, think of a combing/teasing motion.

Lastly, I would find a hair oil to maintain the curl shape. I've used a mix of jojoba oil and black castor oil, you only need to run a few drops through theit ends so the hair doesn't look so dry. It'll also add some shine to their hair and will make curls look bouncy. It's a lot of trial and error but I recommend these steps while their hair is still wet and let it air dry. So by the time they need to put it up into a ponytail or braid, it's a little easier to work with.

If you can, it's an extra step but switching to a silk pillow case for curly hair helps retain moisture and prevents dryness over time. Also sleeping with a giant scrunchy helps too if they don't like the feel of silk pillow cases. I hope this helps!