Kids with difficulty to sleep and wake up

My kids have a strict routine daily. They have the same pre-bedtime routine, and lights out at 8pm every day. We don’t change their schedules, so they don’t ever complain about wanting to stay up either. That brings about sacrifices on our own part. We don’t go out late too. I personally think it’s worth it, and this is only temporary, after all. Building good sleeping habits in the kids helps them in the long run. Each kid (4 and 1yo) sleeps in their own bed, in separate rooms. So no distractions, no noise/fuss.

What I did when my kids are that young. I made a bed time routine for them. Must sleep latest by 8.30pm. I switched off all the lights, I lay in bed and pretend to sleep. I let them cry or be cranky and they went to sleep by themselves eventually. I do that every single day until they can sleep on their own in their own bedroom. My kids are now teens and in primary sch. The bed time routine is still the same even on weekends if we're not going out.