The first question you may have is, "What exactly is a rash guard?" The rash guard (AKA: rash protector, surf shirt, and other names) is a nylon-lycra t-shirt designed to be used in water to protect your skin. Close to the skin so that it remains calm and is not carried around the chest in the water.
In many water activities, such as surfing, climbing, and swimming, your skin is in constant contact with objects in the water and this can cause a painful rash. Even wearing a life jacket while water skiing or snorkeling can cause a rash. You can also buy rash guard online through https://www.coegawear.com/collections/rashguards-ladies-new
Rash guards are a great choice for UV protection because it offers 50+ SPF that lasts all day (in or out of water), doesn't get messy like sunscreen, and doesn't need to be reapplied to your protection all day.
If UV protection is the main reason you are buying a rash guard, it's best to wear long sleeves as it offers the greatest protection and therefore the greatest protection.
When purchasing a rash guard for your little one, consider light colors (say, yellow or red) to make them easier to see in the water, especially if it's a busy day at the beach, lake, or pool.
When it comes to swimming pools, keep in mind that high chlorine pools (i.e. public pools) will jeopardize your protection against rashes because chlorine will eat up nylon lycra (the woman you know you can't bleach your nylon), right?).
If you're lucky enough to purchase a rash guard for this type of use, get one in a color that matches your bathing suit or suit and one size larger. That way, you will look and feel comfortable in style while protecting yourself from sunburn and potentially ruining your trip.