In "Swinging," the speaker is mourning the fact that her daughter is growing up. When injured, mentally or physically, the daughter used to come crying to the speaker (presumably her mother):
She used to fling her anguish into
My arms, staining my solitude with
Her salt and grimey griefs.
The speaker uses the word "solitude" here and this is significant. When her daughter is not with her, she feels alone. The daughter used to cry in her arms thus making her feel not alone, useful. But now that the daughter is older, she remains alone. The daughter is possibly an adolescent and intends to face her own griefs rather than retreat to her mother. In fact, the speaker indicates that the daughter is rebelling: "I, her hatred's object." The daughter is swinging until she is parallel with the sky, going farther away from her mother (the speaker) who is on the ground. The daughter is like a kite soaring away and the mother, wanting her daughter to be closer again, wishes wind the string of the kite back in. The speaker wants to be close to her daughter in order to avoid being alone but also perhaps to protect her. The most striking literary device is the analogy between the daughter and the kite.
Similarly, the speaker in "Nettles" is a parent who wants to protect his son. The son falls into a bed of nettles (sharp needle-like hairs on a plant) and comes crying to his parent (the speaker). This is where the poem differs from "Swinging." The boy is still young enough to always seek security in his parent's arms, whereas the daughter in "Swinging" is trying to make it on her own. The speaker in "Nettles" is protective, as is the speaker in "Swinging." He attempts to cut down the nettles, thereby making the world safer for his child. But the nettles grow back and the speaker concludes that the boy will be hurt again.
Both poems illustrate the protective instinct and love of parents towards their children. The speaker in "Swinging" misses the days when her daughter would always look to her as the protector. In both poems, each speaker realizes that there will be times when he/she will not be able to protect the child. In "Swinging," it is because the daughter is older and trying to be independent. In "Nettles," it is because the world can be a dangerous place and not even the most vigilant parent can protect a child at all times. The emotion conveyed in "Swinging" is more personal: that of a parent's frustration with wanting to protect her child but realizing the child's evolution towards independence. The emotion in "Nettles" is intimate as well but it has more to do with the parent's fears of the dangers the child may eventually face.