Did your LOs start to sleep longer at night the older they got? That is, lie in later in the morning than before?
My hunch is that babies/young toddlers are natural early risers but as they drop all their naps and start pre-school and school they are more likely to lie in after 5.30, 6. 6.30am even.
I am thinking specifically of children from toddlers to 5 or 6 years old (not teens who can sleep for Britain!).