I must disagree, Ariel is completely naked after she becomes human. ;)There are two ways, the Disney way, and the realistic way.
The Disney way, has the clothes adapting and fitting them no problem after a transformation, while of course in the realistic manner, the clothes would fall off, rip, break or whatever the transformation affected to.
In Animorphs, they wear really tight fitting clothes when transforming, I believe. But then, I don't really know what exactly happens. But, animorphs would be the closest thing, I am guessing, so you could try and base it off that.