The poem "We are Seven" by William Wordworth was published in 1798, which makes it a transitional poem from the late 18th century to the early 19th century.
As it is common knowledge, high birth rates were proportional to high mortality rates among infants in the early 19th century and late 18th century, and even before that. There are many reasons for this.
First, most births took place at the family home and very little care was placed on sanitary living conditions mainly because in poorer homes there was probably overcrowding with too many family members living together, no running water, or living in filthy slums where cesspools would overflow and diseases would spread.
Second, disease was rampant as a result of poor hygienic conditions. There was everything from tuberculosis, typhoid (a particularly nasty bout occurred at around this time), and the medical profession had not yet been fully recognized the way that it is today.
For instance, surgeons were still the local butchers, anyone could call themselves a nurse, and it wasn't until much later when Florence Nightingale's efforts to instill high hygienic conditions in hospitals was acknowledged, and the mortality rate did not come to an all-time low but until the end of the 19th century.