A Cry For Help!

I really don't know what to do about this one.  It's a new parenting challenge for me.  My older son has started getting up really early in the morning every day.  By default, my younger son is also getting up very early every day (they share a room).  This makes for 2 very tired, grumpy boys.  My younger one I can get to nap in the afternoon (he's 3), but the older one is in school so there is no nap time.  We have a house rule to not get up unless they hear me, but sometimes they are mistaking their sister's 4:30am feeding for a wake up call.  And I really don't want to shout at them to go back to bed which would only further wake the baby up.  And by the time she's back in bed, they can be wide awake.  They will go back to bed but I know they don't go back to sleep.  Tonight I'm going to put a digital clock in their room and tell my older son what time it's okay to get up at, but I don't know if it's going to work.  Any ideas?  They used to be really good at sleeping until 6:30/7am which is perfect for their schedule.  It's the 5 am that's killing me.  I'd like to blame last week's time change, but really, shouldn't they be adjusted by now?