It's a little bit surprising to me that conservatives have managed to turn this into a subject of controversy, but here's a newish research paper I saw on the National Bureau of Economic Research website that investigates the question of whether giving health insurance to poor children has a positive impact on health outcomes.
The answer is yes—the early Medicaid expansions they looked at had a substantial impact on later-life mortality of black teenagers.
Obviously many people have countervailing concerns about the total volume of federal expenditures in general and federal expenditures on health care services in particular. One could square that concern with basic concern for human welfare by proposing that we shift resources out of provision of health care services to the elderly (i.e., Medicare) and toward provision of health care services to poor families and especially their children (i.e., Medicaid). But the currently Republican plan is to do the reverse and subject low-income children to much more severe cutbacks than the elderly.