母乳で育てています。

。

I'm breastfeeding.

Literal

Breast-milk [with-で] am-raising-[polite-ます]

母乳で育てる = 'to raise on breast milk' — で marks the means. ~ています marks an ongoing habitual action. 母乳 (ぼにゅう) = breast milk (母 mother + 乳 milk).