Ours don't drink squash or juice drinks at home at all, but even we would go with the 'once in a while won't do them any harm' type of approach to squash at parties. Heck parties are full of the type of food that one normally doesn't allow the children but isn't that what parties are all about.....
DB's children not allowed sweets ever in their own home, but if they go to see my parents and are with one set of cousins who live down the road, who do you think are the great sweet thieves? One guess and you would be right!
A little bit of what you fancy does you good and think that most children will accept a "no dear, not at home" if they know they get to indulge a little when out and about. We went somewhere yesterday and Ribena was on offer for all the children attending. We never have the stuff in the house but heck, the whole event was a treat, and all the Jocastas, Harriets, Hugos and Pandoras were doing the same...
Yes, what is wrong with water - mine generally drink just water with one juice a day and DS has a glass of milk because his diet isn't as generally varied as DD. He drinks lemonade if we go out for a meal but DD has no taste for it and is always happy with water or apple juice! Live a little!
It is just funny because it's so precious.... is this an only/first child? Think we all start off very idealistic with our first and then throw caution by the wayside!