just return かんじ　よめません to them.

I prefer use hiragana for how to read kanji, so I would say: ひらがなでかいてもらえますか。 It's the translation, however, we have other ways of say it because of the politeness. ひらがなでかいてもらえませんか。 ひらがなでかいていただけますか。 ひらがなでかいていただけませんか。 (It's the politest between these four.) I hope this was helpful.