Why won't America ratify the UN convention on children's rights?

"This year the spotlight was on the Convention on the Rights of the Child, which has been adopted by every country apart from Somalia, South Sudan and America...Opponents of the treaty say it would usurp American sovereignty, a long-standing fear about the UN among some conservative Republicans. There is a fear that the social and economic rights established by the treaty could provoke lawsuits demanding that the government pay for these things. To address such concerns, America generally ratifies human-rights pacts with the condition that they will not overrule existing laws." (thanks Nabeel)