They are leaning their young heads against their mothers, —
Although the working class had very little of worldly goods, its family unit was quite close. One main reason was they had to share a small space as living quarters. Another reason was children often working alongside their parents. All the children were viewed as a potential source of income so the family strived together as a unit to make ends meet. The close knit working class family was a sharp contrast to the wealthy Victorians. Usually their children were left in care of nannies or governesses. The higher echelon of society had little time to spare for their kids yet had high expectations of them. Even Winston Churchill said he could recall every hug he ever had from his mother.
The difference between the classes here is not immediately discernible for modern readers with just the line describing children leaning on their mothers. In Victorian England, the rich and middle-class did not handle their own children.
Check out https://victorianchildren.org/victorian-child-labor/ for more interesting facts.