A Venn diagram, for example, is a visual language for talking about the relationships between sets. Another way to answer you is to point out that there are many different kinds of both visual and textual programming languages. Some visual languages are very abstract - others are not. Some textual languages support debugging well, others don't. Very preliminary comparisons of Logo and ToonTalk with real kids have been carried out as part of a European research project called Playground.
Early indications are that there are no such problems.