My 4 years old daughter like to drink Coca-Cola and when her grandpa was around she was permitted to drink 100-200ml of carbonated drinks. Should I forbid her drinking soft drinks? What could be appropriate age to consume carbonated or soft drinks? What should be permissible quantity of that drinks?
There is a pretty clear link between soda consumption and obesity. See, for example, https://www.hsph.harvard.edu/nutritionsource/sugary-drinks-fact-sheet/
Awareness around this link is clearly rising and as a result soda consumption in the US has started to decline http://www.businessinsider.com/americans-are-drinking-less-soda-2016-3 Still, the average soda consumption per person in the US is still at whopping 40 gallons (or about 150 liters) per year. Given that many people don't drink any soda at all, the consumption of actual soda drinkers is a lot higher.
Soda is entirely an acquired habit, so the more you can postpone soda in your daughter's life, the less likely it is to become a potentially harmful habit. Ideally you communicate this positively. Bad: "You can only have soda once a week". This makes it something special and desirable. Better: "Soda is really not good for you. Why don't we find something that's good for you and really yummy too. How about ... "
Obviously, role-modeling will help and you need to practice what you preach. You can't make a credible argument that "it's not good for you", if you drink it yourself.
We never had soda in the house and when the kids got eventually exposed to it, they found it too sweet. It seem to have stuck: Now as adults they still don't drink soda.
It would be best to think of sugary drinks like candy - a treat to be enjoyed once in a while. There is no nutritional value in sugary soft drinks, the sugar is just empty calories. Very little nutritional value in sugary fruit juices either.
So I'd not forbid them, but I wouldn't routinely stock the house with such items.
I got to liking unsweetened tea years ago while living in the north of the US. Now I'm back in the south and sweet tea tastes like pancake syrup. All I can taste is the sugar. So liking sugary drinks is an acquired taste.
I think you should try to avoid sugar eating habits in her life. Young children are drawn to fats and sugars.
Instead of giving candies or juices our family tries to find tasty fruits and vegetables. The main reason is to teach good habits.
At previous birthdayparty at our house none of the dozen kids wanted soft drinks. They asked for home made juice.
Bubbly drinks are habits.
I do drink cola or fanta when I'm eating a pizza now and then. But my kids prefer milk, juice, or plain water.
If you are wondering can you advice her grandparents about soft drinks? Sure you can. Suggest a substitute or another way to treat your daughter. Maybe they'll listen to you.
My mother didn't. My mother in law did.