Brief Answer:
Probably dehydration

Detailed Answer:
Hello. I have been through your question and understand your concern

Cramps are prolonged contraction of the muscles to be carefully differentiated from fasciculations, which have other causes.

The most common cause of cramps or muscle spasms in children, especially in the distal extremities is dehydration. The children has a different distribution of fat and water in the body, so sweating, diarrhoea or vomiting produce a important disbalance of water and salt, especially sodium and potassium, a depletion of which cause the cramps and similar features. If this is the case a blood test would reveal the blood electrolyte level and the correction is simple.

The other causes are very rare. Just for information, metabolic disorders can produce cramps, but diagnosis is difficult with some very specific blood test which measures certain enzymes in the blood.

The third cause is a neurological condition called myotonia and the diagnosis requires electromyograghy and an expert neurological consult.

The last two diagnosis are very rare, but you should just consider them without much concern.

Hope this helps.

Wish your niece the very best health

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