I am reading Philosophy in the Modern World by Anthony Kenny, and he writes on page 22: "For the rest of his life he lived in London, often in abject poverty, which caused three of his six children to die of starvation."
I would love to know more about Karl Marx's life in London during that period.
Karl Marx life in London is detailed in Fancis Wheen's biography. Henrich Guido Marx died at the end of 1850 suddenly after a fit of convulsions. In Easter 1852 shortly after her first birthday Franziaka had a severe attack of bronchitis. At first the family could not afford to hire an undertaker but a neighbour took pity on them and lent them two pounds. These deaths took place in the first two years Marx was in London, had no reliable income and was living in very cramped flats. Engels attributed the deaths to the conditions the family was living in and Marx showed great affection for his children. Given the period and the comfortable lifestyle Marx provided for his family as soon as he was able I think it is safe to attribute the deaths to poverty.