Why is it difficult to lobby for the interests of poor children?

Because the children don't vote.

Explanation:

Lobby, in politics, is, by definition, any practice practiced by people or companies to influence the decision-making spaces of the government, acting in both the Executive and the Legislative, being more frequent in this last mentioned sector. Basically, lobby is a way of communicating, debating, or trying to convince parliamentarians or government officials (as well as close officials, such as advisors and secretaries) to make a particular decision to address particular or general interests.

We can say that lobbying is an exchange between a group of people and parliamentarians, as these people promise to support parliamentarians and vote on their candidacies if the group's interests are met. However, as children do not vote and are unable to exert any political influence, it is difficult to lobby for the interests of poor children.

because poor choldren are very poor