I have travelled to a number of locations where the tap water is unsuitable for babies and toddlers (and adults for that matter) to consume. A question of concern for many parents is ‘can babies drink bottled water?’ From my research and experience it appears that young children can drink bottled water so long as the sodium level is under 200mg per litre (20 mg per 100ml). This information is nearly always found on the side of the bottle so you don’t need to worry about packing a chemistry set in your carry-on. If this information is missing then I would recommend finding another brand. Most internationally distributed brands of water are safe to give to young children and many of these brands also include a ‘suitable for infants’ endorsement on the bottle.
Be aware that bottled water labelled as natural mineral water or spring water can actually contain very high levels of sodium. For instance, I recently came across a number of local brands in Egypt that had a sodium level greater than 50mg per 100ml! You should also always check that the tamper proof seal is intact on the water you have purchased. It has not been unheard of for dodgy shopkeepers to fill used, branded bottles with local water, passing these off as new to unsuspecting tourists.