Although most sunburn is not too serious, they do cause great discomfort and should be treated properly.
Here are some tips to treating sunburn:
Make sure to give your toddler plenty of fluids, to replace fluids lost by being out in the sun.
Soothe the sunburned area with a clean soft washcloth soaked in cool water. Keep the cloth on the burned skin for 10 to 15 minutes a few times a day.

Give your toddler a cool bath. Add baking soda or an oatmeal-based bath treatment to the water. Make sure that you pat your toddler's skin dry. Don’t rub the skin with a towel.
Apply a water-based moisturising lotion that does not contain alcohol, or an aloe vera gel to relieve itching,
Do not use petroleum-based products like petroleum jelly on your toddler's skin. These can actually make sunburn worse because they prevent heat and sweat from escaping. This is also true for butter and oils.
Do not use first-aid sprays or ointments with benzocaine. The benzocaine can irritate your child's skin or cause an allergic reaction.
Do not use ice or ice water on your toddler's skin.
Never intentionally pop any blisters. Blisters form to protect the underlying skin. If you break them open it can lead to infection. When a blister does break, gently trim off the dead skin and apply an antibiotic ointment.