FOR the record, it can be lightened. A polishing of the internals will make the trigger smoother and feel lighter. You cna also get aftermarket spring kits to lighten them (allot). The lighter spring kits can cause occasional FTF though. ( Not advised on a carry gun) Mine has been smoothened out and is VERY nice.
Having said that, I agree with the recomendation for a .22. Teh 640 is not a novice friendly gun to plink cans with. A 22 gives less recoil, more accuracy and a higher comfort factor for kids getting started. Besides everybody needs at least one!