Without taking a stance on the question of child sexuality - what you say is true, but is there any particular selection pressure for it to be off, either? Evolution goes for the simplest solution, and "always on" seems to me simpler than "off until a specific age, then on". Of course, that's an oversimplification. The required machinery may simply not be developed yet, in the same way that you need to first grow to be four feet tall before you can grow to be five feet tall. But then, when you reach the size of four feet, you already have four fifths of your five feet-tallness in place, so it stands to reason that that at least part of what makes sex pleasurable will be in place before adolescence. Whether it's active is obviously a separate question, but I don't think "has no particular evolutionary reason to be active" tells us much by itself.