What are these two characters marked red? I'm trying to learn reading some kid books, some Bonobono font chars meaning though is a mystery for me.
To actually answer the question, the characters are な (na) and ぢ (pronounced ji, but di in certain romanisations).
To address the meaning of what's being said, I understand that there is a slight difference in the meaning of いじめる and いぢめる, both meaning to bully/tease.
いぢめる has some sense of cuteness/playfulness about it: perhaps you're teasing someone, but you don't really mean for your words to hurt them -- you might use the kanji 苛める.
いじめる on the other hand has the true meaning of bullying about it: more hurtful and unpleasant -- you might use the kanji 虐める.
Note, however, that the (strictly) correct 'spelling' of both of the above verbs in modern Japanese is いじめる.
The use of ぢ in いぢめる is a historical spelling of the verb, before ぢ was (largely) rationalised to じ in the 1946 script reforms. Both じ and ぢ are pronounced the same, except for I think in certain dialects in Kyushu and Shikoku.
Note, the word is いじめる and is normally never spelled いぢめる. It seems spelling it that way is a quirk of this character.
(Normally じ and ぢ would be pronounced the same (ji), but in this case it’s possible the artist was going for something more ‘squirrel-like’ in pronunciation, like an emphasized/partially-voiced ち.)