oh! John, catch haud o’ him
Johnston keeps up a strict rhyme scheme throughout the poem - aabbccbb, etc. However, the speaker's somewhat fourth wall breaking exclamation here tips that rhyme scheme on its head. Blin', in the Scots dialect, is a near rhyme with the word him, though that rhyme is lost some in other accents. Beyond the loose rhyme here, the outburst also changes the otherwise even flow of the rhythm through the poem.
The thought of her child falling to the floor forces the speaker out of her careful patterns, highlighting the mother's love and care for her children.