How well do black out curtains work?

Koda

My 3 year old has a tendency to get out of bed and play during nap time. Of course we have other solutions but I've realized that at night their room is so pitch black that they can't see enough to get up and play so they stay in bed.

They only have one window and almost no light can get under their door. If we got black out curtains would it make it so pitch black that they wouldn't be able to see in order to play?

Just an idea.