Prononcer is just fine even with kids but beware that unlike with English, you cannot say that a letter say or read something. The pronominal (a.k.a reflexive) form (se prononcer, se lit) is mandatory here, e.g.:

La lettre C est une consonne qui se prononce soit comme un S, soit comme un K suivant la voyelle qui suit. Les deux lettres AU se prononcent comme un O.

The pronominal form is mandatory when letters, syllables, words and sentences are the subject of prononcer, but otherwise, the direct, non-reflexive form is used:

Les anglais n’arrivent pas à prononcer correctement les mots serrurier et croissant.

Vocaliser would be less understood, especially by kids, and might also be used to describe singers exercising their voice. Articuler is specialized, and is more about the quality of the pronunciation than the pronunciation itself.

Se prononcer is the best translation

However, in a less formal style (similar to “the letter says”), you could also use :

se dire

La lettre C se dit 'K' sauf devant y, i ou e.