Single - dependent of parents - child EIC?

Assuming the $2500 is earned income, you are correct that for purposes of the EIC, the infant remains the qualifying child of the 19 year old parent. This holds because the 19 year old is not the qualifying child of her parents. She is allowed to be a qualifying relative of her parents and still get the EIC.

Note, that you can get a different answer, if the 19 year old does not file a tax return or files ONLY to obtain a refund of taxes withheld. Under these circumstances, the infant would not be the qualifying child of the 19 year old parent. The infant would be the qualifying child of its grandparents and it is they who might be eligible for the EIC.