In bi-shiqqin, bi- is a preposition and shiqq is its genitive complement, meaning one side or half of the body. Wa-taḥtī shiqquhā forms a nominal construction, with the locative phrase as an advanced predicate and shiqquhā as a delayed subject.
In lam yuḥawwal, lam negates and governs the jussive, while yuḥawwal is a passive imperfect of Form II, meaning that the body’s other part was not moved away. A final vowel may appear in poetic recitation for rhyme, without changing the underlying jussive relation.
The grammatical arrangement heightens the contrast between the two halves: movement toward the child versus remaining near the speaker.