In the sources below are several good articles about gluten allergy and gluten intolerance. The two are sometimes used interchangeably, but technically speaking, in allergies, the immune system is involved, whereas in intolerance, it is not.

As for symptoms, according to http://www.livestrong.com/article/8949-spot-signs-gluten-intolerance/ :

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Getting a gluten intolerance diagnosis may be hard, as symptoms of gluten mimic several other ailments, making the cause overlooked by many doctors. Signs include diarrhea, fatigue, stomach cramping and gas. Without treatment, skin rashes and weight loss may also occur. While similar symptoms occur in both adults and children, it is sometimes easier to spot the early signs in young people. Constipation usually occurs only in children. More importantly, infants, toddlers and children who do not gain weight should visit a doctor as soon as possible. This condition, called a failure to thrive, indicates malnutrition of some kind. Food intolerance, including gluten, is a likely cause in an otherwise healthy and well cared for child.
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according to http://www.livestrong.com/article/32512-wheat-gluten-allergy/ :
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Symptoms are not the same for each sufferer but may include bloating, stomach or intestinal cramps, headaches, vomiting, diarrhea, muscle cramps, swelling in or around the mouth, anemia, allergic rhinitis (sneezing or runny nose) or hives.
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The Mayo Clinic has an article on wheat allergy, which includes gluten allergy, at http://www.mayoclinic.com/health/wheat-allergy/DS01002/DSECTION=symptoms . According to this article, symptoms in children typically show up within a few minutes to hours of eating foods with wheat in them (e.g. bread). These symptoms might include e.g.:

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* Swelling, itching or irritation of the mouth or throat
* Hives, itchy rash or swelling of the skin
* Nasal congestion
* Itchy, watery eyes
* Difficulty breathing
* Cramps, nausea or vomiting
* Diarrhea
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In extreme cases, the allergic reaction could become life-threatening (such extreme allergic reactions are called anaphylactic reactions or anaphylaxis, and include more typically reactions to e.g. bee stings). In these severe cases, the person suffering the reaction might exhibit one or more of the following symptoms:

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* Swelling or tightness of the throat
* Chest pain or tightness
* Severe difficulty breathing
* Trouble swallowing
* Pale, blue skin color
* Dizziness or fainting
* Weak pulse
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