What are some other negative consequences if a child won't follow the sleep rules?

As mentioned in the course, securing the door in the moment when the sleep rule is broken is the most effective natural negative consequence. It is immediate and it has the effect of removing all attention from the situation.

If you prefer to try a different consequence, you can do Silent Return with NO limits. Learn more: https://awesome-little-sleepers.helpscoutdocs.com/article/90-silent-return-without-limits

For other options, think of what might be a natural consequence of you and your child not getting enough rest the next day. Maybe a planned trip to the park or a play date would need to be cancelled because everyone is too tired and grouchy.

Or, especially for older kids, you can consider using what I call their "currency" for a negative consequence. What do they value most? One of my kid's currency is TV, the other is her scooter. So, I might prohibit use of one of those things the next day.