Knowledge is power, and I assume you know the limit that your child can have.
Dasani:
"although we are unable to disclose the exact quantities of minerals added to our water, we can tell you that the amounts of these minerals (including salt) are so miniscule that the US Food and Drug Administration considers them negligible or ‘dietarily insignificant’."
Dasani ADDS those things to the water.
Evian:
Electrolytes & Mineral Composition (mg/l)
Neutrally balanced pH: 7.2
Calcium: 80 / Magnesium: 26 / Potassium: 1
Bicarbonates: 360 / Sulfates: 14 / Chlorides: 10 / Silica: 15
evian has naturally occuring elecrolytes contributing to the taste nature intended.
With Evian the minerals are naturally-occuring. You're going to get those in any water unless you drink distilled, which would cause its own issues, I imagine.
And if the sensitivity is that much of a problem, then all food would need to be cooked in distilled, etc... So I assume you know the amount that's safe in a day.
I personally like Zephyr Hills water, and always head to the store to get a ton of it. We have an Igloo cooler backpack that we load up, and we know we've done well if it's almost empty at the end of the day. If we had a big group we would take both of the ones we have.
You can go to their site and get the pdf of their water quality; they have the sodium and potassium listed. (unlike Dasani)