Infants (below 2 years old) do not count as fare paying passengers. They are not entitled to a seat, which is a moot point in a sleeper. There are specific ratios applicable to adult to child and infant ratios that may cause a child to be ticketed as an adult, or an infant as a child, but again, that is not your situation. Unless dealing with unaccompanied minors or border crossing, I am pretty sure Amtrak ID policies only apply to passengers above 18 years of age.