info below taken from :
http://en.wikipedia.org/wiki/Oxalis

looks like the answer to your question will really be found in the 2nd paragraph.

The edible tubers of the Oca (O. tuberosa), somewhat similar to a small potato, have long been cultivated for food in Colombia and elsewhere in the northern Andes mountains of South America. The edible leaves of Scurvy-grass Sorrel (O. enneaphylla) were eaten by sailors in southern South America as a source of vitamin C to avoid scurvy.

A characteristic of many members of this genus is that they contain oxalic acid, giving the leaves and flowers a sour taste, refreshing to chew in small amounts. However, in large amounts, these species are toxic, and interfere with proper digestion. In the past, it was a practice to extract crystals of calcium oxalate for use in treating diseases and as a salt called "sal acetosella", or "sorrel salt" (also known as "salt of lemon").

Several species are grown as ornamental plants in gardens, while others, notably O. pes-caprae and O. corniculata, are pernicious invasive weeds in cultivation away from their native ranges. A species which regularly has leaves with four leaflets, O. tetraphylla, is sometimes misleadingly sold as "four-leaf clover", taking advantage of the mystical status of four-leaf clovers.

Your daughter isn't doing herself any favours. Oxalis is very high in oxalic acid, which is toxic to humans.

Almost all green leafy foods contain oxalic acid in some strength or another, even lettuce and spinach. But you'd have to eat a lot of these things to get oxalic poisoning. Oxalis and sorrels are a different matter. In very small, infrequent amounts they probably won't do much harm, but in frequent, or large amounts they can be very harmful. It's the large amount of oxalic acid in rhubarb leaves which make them poisonous.

Eating large or frequent amounts of oxalic acid can lead to kidney damage. Symptoms of poisoning include:

convulsions, collapse, abdominal pain, low blood pressure, shock, tremors, vomiting, mouth pain. Severe damage to the mouth, gastrointestinal tract, or airway may occur and quickly cause death if not treated. Oxalic acid also makes calcium unavailable to the body - not a good thing for a young girl who will need plenty of calcium now, while she's still growing, and later, when she could be a risk of osteroporosis.

For foods which contain high amount of oxalic acid, it's best to cook them like spinach, in several changes of water.

Oxalis has no known medicinal uses. That makes it pretty useless as a plant, given its toxicity.

Discourage her. She might like French or Wood Sorrel, but she should eat these sparingly.

Thanks Renya and Daisy-
I appreciate the feedback. I'll let my daughter know. I doubt she is consuming enough to be harmful, but it's best to be safe! Kathy

If we stopped eating all of the foods that contain oxalates, we'd practically starve, lol! A nibble of fresh oxalis won't harm your daughter; I still enjoy that tang to this day.

How about a diet restriction of any high cholesteral foods, no potassium based foods, no sugar foods. You don't have much left...