A lesson in politeness. おしっこ is the childish word for 'pee'. お手洗い ('honorable hand-washing place') is the polite euphemism for 'bathroom' — it originally referred to the hand-washing basin at a shrine and was extended to the whole facility. ~んですが is the softening polite explanation ending. ~なさい is the soft imperative. The contrast demonstrates register shift from childish directness to adult formality.