。
A child learning to speak isn't formally studying.
Literal
Speaking [nominalizer-こと] [object-を] learning child [topic-は] learn [it's not that-わけではない].
Plays on two senses of 学ぶ: the first (学んでいる) means 'acquiring naturally,' the second (学ぶわけではない) means 'studying deliberately.' ~わけではない denies the formal interpretation. The contrast highlights how language acquisition differs from academic learning.