The title initially causes conflicting feelings, in the first series the behavior of the child characters seems stupid and illogical, which is annoying. At least some interest is supported by the Abyss itself - I wonder where it came from and what is there at the bottom. There is a fear that this will be a cheap tracing from Journey to the center of the Earth with almost chibi-like characters.
Closer to the middle, it becomes clear that the series is not as simple as it seems. The abyss is quite worked out, and sweetness concerns only graphic style. The contrast of the visual style with the events that are taking place sometimes causes a very terrible sensation, which is only aggravated by the realization that almost all the characters here are children.
By the end of the season, the degree of what is happening constantly increases, alternating with relaxing scenes. Screenwriters as if deliberately trying to catch a relaxed viewer.
The landscapes are painted perfectly, the Abyss seems worked out, the soundtrack is very pleasant. We'll see what happens next.
The Beginning of the Journey and Wandering Twilight is the recap of the whole season. Of the extra scenes, there are a few minutes at the beginning of the first film and an extended scene at the very end of the second.