Not true, I never, ever felt tired at "bedtime" as a kid, ever. I would protest my bedtime pretty much everyday because "I'm not tired." Fixed bedtime still "worked" for me because when you're 1) on a firm schedule and 2) laying in bed without anything else to do then sleep comes.

I would (perhaps naively) think that children are the same as adults in the regard that we both need ample amount of time to start slowing down and relaxing before bed time.