Run down your model's battery during the day? Tire them out enough to make them want to sleep?
What's even more crazy is that often when I attempt to silence the low battery warning by placing the Small Child in the charger, it often refuses to begin charging until the battery has been entirely depleted, so along with not being dismissable, even attempting to charge the Small Child does not stop the low battery warning consistently! I believe this is likely a bug, but I haven't yet been able to consistently replicate it to file a bug report despite experimenting with various charging and usage schedules.