That is disputed. The evidences are unclear -- those cremated babies could either be sacrifice, as is generally believed, or they could merely be a religious practice dealing with the remains of babies that died by other means, after all the ancient world's mortality rate for young children were extremely high -- and the only written records of such practice came from the Romans. They had every intention of vilifying their mortal foes
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Like you said, it's generally belived that there was sacrifice going on. I am one of those adherents to that theory. I must confess that I do find the arguments against it not fully convincing.

Rome wasn't the only culture that engages in barbaric pitfighting.
I think that few cultures would build a such construction to pitfighting, and even less likly call if Colosseum (sp?) if they were not Romans.