Chemically tasting Milkshake

My kids love another brand of chocolate milk shakes and strawberry ones. It so happened that my son went on a school outing and I needed to pack him a lunch bag for this outing. I purchases a couple of chocolate milkshakes and strawberry ones as I have 3 kids and we drink it as well. my son came home after the outing complaining he had nothing to drink the whole day as his milkshake is not nice something is wrong. I tasted it and it is not sour but has a chemical like taste to it. the use by date is 28/11/2015. very disappointed in this product as I use to purchase it a lot but cant take a chance if it is going to cause health problems with my kids. I don't keep my slips when purchasing things so I cant even take it back to the Shoprite where I purchased it from. makes me think twice before purchasing this product and my kids love it cause of the Disney characters on it. now how do I explain to them that this milkshake is not good for you? not going to waste money again on this as it is not the first time I bought this product and it is off.