That’s a directive, it’s not a regulation, and the regulation calling anyone under 18 a child does not mean that everything under 18 is treated the same way in actually applicable law, which directives very much aren’t. Germany, for example, splits the whole thing into under 14 and 14-18.

We certainly don’t arrest youth for sending each other nudes:

(4) Subsection (1) no. 3, also in conjunction with subsection (5), and subsection (3) do not apply to acts by persons relating to such youth pornographic content which they have produced exclusively for their personal use with the consent of the persons depicted.

…their own nudes, that is. Not that of classmates or whatnot.