The shirt protects from the sun, pants not (ok, a little).
I think it is well tolerated to let small children go in their birthday suit if they want, always keeping an eye at they and the surrounding. As far as top-less for girls go, there should be no problem as long as they don't have any thing diferent from boys there, but if she want the top, of course give it to her.
This is for "fully clothed" beaches. Where top-less or nudity is tolerated for adults, obviouly kids can go in the same way. Shame of his own body is something learned, and we go as far as puting clothes that don't hide almost anything to enter in the water and trap sand and humidity.
