Roll on or makeup application brush. The latter especially for the face.
Rash guards are great for sun. They are not great for swimming in water especially with young kids. They keep them too cold. Fine for 2 months out of the year when its 90+ but fyi not great for most of the year or if there is a breeze.
"A typical rash guard worn alone, made of spandex, nylon or polyester will not provide a lot of warmth. This is because it is not thick or strong enough to work like a wetsuit, which keeps a layer of warm water close to the skin. However, a rash guard under a wetsuit can provide an extra layer that helps trap that warm water.
Read more about how wetsuits keep you warm in our Ultimate Wetsuit Guide.
A dry rash guard is great at keeping you warm when you have finished a dive. This is because it will help dry your skin quicker, making you feel warmer. Some rash guards can also be used as a wind barrier. This stops the cold air from chilling your skin, making you feel warmer too!"