Polycarbonate water bottles (labeled #7) contain bisphenol A (BPA), which leaches from the plastic even at room temperature and has been linked to chromosome damage and hormone disruption. These are the types of plastic Nalgene water bottles found in sports stores.
#1 PET or PETE bottles (polyethylene terephthalate) may leach DEHA, a known carcinogen, if used more than once. Additionally, your water bottle that has been refilled is likely to contain potentially harmful bacteria that grow on saliva, food particles, and fecal material from unwashed hands. Many people have reported getting diarrhea from their reused water bottles. Washing bottles with hot water and detergent or a rinse with bleach will sanitize them, but also leaches harmful chemicals out of the plastic.
Use metal bottles. A Swiss company, SIGG, makes colorful and safe aluminum drinking bottles lined with an impermeable alloy. They can be found on the Internet. The best price and the greatest variety of choices I have found in the US is at www.momovelo.com in the sundries/liquids section. They offer both children's and adult's sizes and designs. Many other sites carry SIGG bottles as well. Thermos makes stainless steel bottles for both hot and cold drinks or soups at www.thermos.com. Wash these bottles and their tops with hot water by hand after use. Dishwashers may not get into the narrow mouths of water bottles to clean their interiors.
Safer plastic containers are polyethylene #2 and #4 (High density HDPE #2 and Low density LDPE #4). HDPE is used for milk, water, juice, and yogurt containers. LDPE is used for squeezable bottles (honey, mustard, etc.).
Buy food products packaged in glass whenever possible. Use glass containers for storing leftovers.
Do not reuse plastic water bottles.
And don't forget to have children wash their hands after using the bathroom.