The best thing your child can drink is water. It's the healthiest thing for them (as a perk, it also happens to be the most inexpensive). Tap water is great and in most communities has fluoride (~7 parts per million) - as a heads up if you don't want your child having fluoride from the tap: Brita filters do NOT filter out fluoride (you would need to get a reverse osmosis water filter). White milk is great also.
What's our take on juice? It sucks. So much sugar, minimal nutritional benefit (even the "healthy" kinds of juice that are 100%juice like Juicy Juice and Mott's). No child under 2 needs ANY juice. If you desperately want your child to have juice, keep it to no more than 4oz/day (1 small juice box).
Other drinks that are not good for your child's teeth: flavored milks like strawberry or chocolate milk, sweetened teas, other sweetened beverages like fruit punch, lemonade, etc.
What about seltzer? It's water so it's gotta be ok, right? Unfortunately, not in most cases. While it is better than soda or most sweetened beverages, the fact that it is carbonated oftentimes means it is acidic, which is bad for your teeth. If you/your child drinks a lot of soda, we recommend switching to seltzer as a way to stop drinking soda, then gradually switch from seltzer to plain water. Perrier is the least acidic and San Pellegrino is also not as acidic as most seltzers.
Good: Water, White milk, seltzer in moderation (Perrier/San Pellegrino preferred)
Bad: Juice, flavored milk, sweetened teas, sweetened beverages like fruit punch, SODA
The way cavities work is they need time and the right environment to fester. That environment happens for at least 30 min every time I eat/drink anything that causes the pH level in my mouth to drop. So if I am constantly snacking/sipping a drink (even if in small quantities), my mouth never gets a chance to rest and it's constantly in that cavity-causing environment.