The internet is your friend, everyone. Don't just guess, don't just assume - do your research!
About whether or not it's okay if the child says yes -
What we are looking for here is not just 'consent' but 'informed consent'. 'Informed consent' is the only real acceptable form of consent, because all people, especially children, make impulsive decisions out of naivete without having the experience to ask the necessary questions to look out for their own self interests, their own happiness, before they give consent. Unless someone gives informed consent, inevitably they will end up feeling like they've been tricked into doing something nobody told them they wouldn't like. For example, if you think that it's okay for an adult to have sex with a prepubescent child as long as the child says yes, would you be okay with a third grader being hired for a job as long as the third grader says yes? Do you think they should be held accountable to the contracts they sign, even though the contracts are written by lawyers at a high-school graduate level while the third grader is still reading short chapter books? Prepubescent children are not on the same level as sexually developed adults. This is a fact. And they cannot make informed decisions with sexually developed adults, because they lack that emotional and sexual understanding about those kind of relationships.
Some children show a lot more maturity than others, and even understand more about sex than many teenagers do. But the reason these children catch our attention and we call them 'beyond their years' is because they are the exception to the rule. It's not fair tailoring laws to the majority of children based on these particularly mature kids.
About why pedophilia is a disorder -
The simplest answer is that it presents a risk factor to its subject, children. Homosexuality, in comparison does not present a risk factor to its subject, the same sex.
If a person is attracted to children but never feels the need to act on it, then many people do not consider them pedophiles. On the other, there is a proposition going around to remove 'pedophile' label from the list of disorders and uses it to describe those who are not sexually active with children, but say that people who actually molest children have 'pedophilic disorder'.
Additionally, there are other symptoms connected to the disorder, such as a social disability in the form of not connecting with adults and being more comfortable with children as company because their development is at a stunted level comparable with a child. Sometimes people who developmentally delayed never really mature beyond a certain point in their childhood, so they look to kids instead of adults to fill the role of their friends and sexual partner, making them at more risk for molesting kids. I've actually had this happen close to me by people that I care for.
The real information here is really complex and has a lot of ins and out and exceptions, and I see a lot of people reaching really disappointing conclusions because they want to make these clean, straightfoward categories that are completely ridiculous. Don't come into a discussion if it's too much work for you to research it and learn the ins and out of it! This is just common sense in any discussion.