Two short sentences, two images, and not a single word of explanation. The speaker puts on a green nightgown and says they sleep "light as a child." That can mean sleeping gently and easily, without worry, but it can also mean sleeping lightly, close to the surface of waking. The poem leaves both readings open.
The second image shifts things. The speaker moves "through the night / like the remains of a fire." Embers are what's left after the blaze is over: quiet, low, still warm, and slowly fading. Set beside the child, the comparison feels a little bittersweet. There's a hint of youth and of age in the same moment, someone who is both innocent and worn down.
The mood is hushed and calm, with a faint sadness underneath. Nothing dramatic happens. The poem just lets us watch someone drift through the dark, glowing softly, neither fully asleep nor fully gone.