In general, you would give a child about half the dose an adult would take.
Children come in a whole range of physical sizes, so this is relative and judgement should be used. A large 15 year old may be a big as an adult and able to take a whole dose, and some adults can be no larger than a child, and so would not need a full dose. The amount of burn should also be taken into account.
The dosages are based on average adult sized body, with sunburns that would usually take a week to handle.
An ‘average’ adult would take one packet the first day and another 12 – 24 hours later. An ‘average’ child would take half that. Again, if a burn is very severe and medical attention is in order, then that is what should be done.