Normally children would have been enslaved or just killed quickly if they were too young to be enslaved, and that would have happened only in a mass conquest situation or something equally unusual.
Normally, there would have been no point to crucifying children. The Romans were efficient if nothing else, and they weren't gratuitously cruel (or at least, gratuitously willing to infuriate the natives. They wanted to co-opt them, not anger them.) Crucifying babies wouldn't have represented their modus operandi in most circumstances. So, quick deaths would have been preferred.
Where did you read this? In a work by a serious historian? If so, who?
However: there was the long siege involving Titus and his laying waste to Palestine in 70 AD in which entire Jewish families were apparently crucified, but that was unusual.