Our parish published guidelines insisting upon covered shoulders and tea length dresses. They said sleeveless dresses should have cap sleeves sewn on or a shrug or bolero jacket should be worn over during mass. The material said that as always, we should be modest before Jesus.
I was thankful for clear guidelines. It took out a lot of the guesswork. And it was a subtle reminder for the parents that we should always be properly dressed at mass. We live in the Deep South, but we have an air-conditioned church. So we went with a beautiful sleeveless style with wide shoulder straps, combined with a matching bolero to be worn in the church. Modest AND comfortable.
That being said, I do not see ny problem with tank tops and swimsuits in appropriate contexts, no matter the age of the person. Part of modesty is dressing appropriately for the occasion. A beach party is far more casual than a Mass, and beach attire would be immodest and inappropriate for Mass, obviously. That is an extreme example, but my point is just that we cover more of the body in more formal and reverent occasions. But little girls should NEVER be made to feel ashamed of their bodies. They should be told that we veil that which is holy, and we protect that which is private or vulnerable. This is why we drape our bodies with more fabric in situations with more people, more reverence, more importance. So that we can keep the focus off of ourselves and allow us to focus upon our Lord without being distracted by anyone gazing at us. At least this is how I have attempted to explain modesty to my daughter. I am especially concerned about her becoming ashamed of having a female body, so I am very careful to discuss it in a positive way.