The key link between these two poems is the way that both of them focus on either a near-dead or a dead child that has needlessly died. Also, in addition, you might like to draw a comparison between the settings of the two poems and how terrible they are and the way that the poets use explicit religious comparisons to compare the children in order to write a poem of protest.
Consider first of all the children that are described. In "Refugee Mother and Child," the love that the baby's mother has for him is shown in the way that she combed his hair and then began to part it. However, the poem somberly ends by remarking that she did this as if she were "putting flowers / on a tiny grave." In "An Abandoned Bundle," the unwanted child is already dead, and is described as "a squirming bundle," and then "a mutilated corpse," with these descriptions really serving to underline the inhumanity of this act of abandonment.
In terms of setting, Achebe's poem is set in some kind of famine location in a hospital or makeshift refugee shelter. The air is awash with the "heavy odours of diarrhoea and unwashed children" with their ribs painfully sticking out. This is a scene of death and destruction. In Mtshali's poem, the setting is a rubbish heap on the side of a village where the morning mist:
flowed thick yellow
as pus oozing
from a gigantic sore.
In both poems, note how the setting helps to develop the way that the poem is a poem of protest, trying to make their audience sit up and realise their own inhumanity.
Lastly, note the religious comparisons. Achebe deliberately compares the mother and her baby to the Madonna and Jesus, whereas Mtshali compares, in a very dark ironic fashion, the abandoned bundle to "Baby in the Manger" who slept on "human dung."
These are the three elements I would start to use to comapare these two excellent poems. Good luck!