The majority of Aztec sacrifices were slaves and prisoners of war. However, children were a bit of a special case. Several Aztec festivals required the sacrifices of children in particular (primarily festivals related to water). Some of these children were slaves. Others were purchased from their mothers for the purpose of the sacrifice. Sacrifices to Tlaloc were chosen specifically from the Mexica population based on favorable signs and there is some evidence that they were the children of noblemen. If you are looking for relevant primary sources, Fray Diego Durán's, Toribio de Benavente Motolinia, and Bernardino de Sahagún all documented their observations of the festivals and sacrifices. (Durán in particular has a lot about sacrifices to Tlaloc.)

The confusion is that Mexica, or Aztec, society was not ethnically homogenous. As any big city in the ancient world, it was made out of conglomerates of people coming from different regions that had emic views to differentiate themselves in different ways that we in current times. For example, while they have a Mexica (which actually some of them called themselves Culhuas, meaning of people from Culhuacan) label for identity, their most important allegiance was to their Altepetl, not to their ethnic background. For example, while you may have people that ethnically considered themselves similar, they could have differentiated from each other based on allegiance to different altepetls. And since Tenochtitlan was a complex Altepetl, made out of smaller Altepetls, this means that you could have had people from ethnically distinct backgrounds serving under the roof of an Altepetl that was part of the Mexica control altepetl that we label as the city of settlement of Tenochtitlan. My point here is that our labels to differentiate ethnicity were not the same as those of the Aztecs, causing referential problems.