There are reasonable arguments that a consent must not be verbal. Indeed, most emotional communication is nonverbal or is using a verbal form only formallly. It is easy to say "I love you" so that it means "I hate you", the reverse is also possible. Nonverbal does not mean uncertain. Often body language and intonation is much more certain than formal language.
Moreover, a child which does not really like to have sex, but verbally agrees with sex because of some other reasons (fear, respect, material reasons) often expresses his disagreement on the nonverbal level. Thus, it is necessary to "here" such nonverbal statements. To a pedophile who loves the child this is usually needless to say.
On the other hand, relations which use only nonverbal communication usually do so because of some problems in this relation like fear of talking about sex. If the relation is ok, why not to talk about it? Usually this question points to some problem of the relation, for example one or above participant think that what they are doing is "dirty".
Another argument against is that misunderstanding is more probable. It is simple to express own feelings and wishes like "I (don't) love you" and "I (don't) want to make sex with you", "I'm afraid of detection". But to explain more complicate things like "I want to make sex only if you want it.", "I love you even if you don't agree to make sex.", "I'm afraid of detection not because we are doing something wrong and dirty, but because it is forbidden by some ... law." is complicate if not impossible. But such things have to be clear for the child, else there is no real consent.
Last not least, the impossibility to speak about sex is one of the main problems people have with sex. It is mainly caused by the sex-negative upbringing and the general sexual repression which is widely distributed in our society also after the "sexual liberation". If the adult is able to speak about sex with the child, this may have a great positive influence on the child.
To have somebody to talk about sexual problems is one of the main benefits for a child in a pedosexual relation. There may be discussed as problems of the relation itself (the necessity to hide it, guilt feelings, information about the law and usual police tricks), as general sexual problems (other experiences of the child, f.e. pornography, problems of the child with own homosexual or other "perverse" feelings).
Because of this reasons, I thing that a "nonverbal" relation has a much lower ethical value compared with a relation which includes talking about sex. On the other hand, nonverbal contacts are not a-priori bad or inacceptable.
There is one interesting reason for the usage of nonverbal communication - it allows much smaller steps in a step-by-step method of establishing consent. Using language, you can talk with the child about sex, and at some moment you can ask it if it wants to make sex with you or not. "Body language" allows much more intermediate steps. Thus, many pedophiles use it for security reasons. If you are accused to have made an "improper touch", you have a good chance saying that this was a misunderstanding or accident. If you have explicitely asked a child for sex, your situation will be much worse. Even if such an attempt does not lead to law persecution, you are clearly disclosed as a "pedophile", and this will be harmful enough.
Thus, to use nonverbal ways instead of verbal is also evasive behaviour - a negative side effect of the age-of-consent laws.