It depends on what your aim is, but the problem still requires you to deal with the children that Shizu presents to sir Slime. I mean sure you could have made it a roundabout way to expand his Country, but I am unsure how I would have written it differently. It may seem like its weird because it's only one season. I think the ways stories are told it normally reaches a plane where it's just a neutral zone nothing is really happening, this was more of a wrapping up of small things that weren't addressed because of the fast pacing that it started out with. I think the problem will fix itself with a second season. I do agree it's uncommon but not unusual, I mean we see it in The Hobbit(LOTR) and Starwars where the Climax is told first and then the anticlimatic stuff is told afterward. I am of the view that a similar style was attempted here, but people had different expectations.
I mean, you say it's just one season, but it's actually 2 cour. If you have the climax after 15 episodes or so, let the rest of the story just simmer out and throw some gaiden at the end for good measure, then there is something seriously off.
There are several ways how you could have solved it without changing the source material:
- Don't rush in the middle part like they did, flesh it out and make the last arc more concise.
- Put the gaiden stories in the middle and have the climax later, use a more concise children arc as an epilogue.
- Switch it up. Do the children earlier and the arc before that as a season finale.
ecetera.
And they didn't even had to wrap that up at this point. Shizu's story is not over until he confronts that demon lord, so there was no reason to rush the children's arc into the first season. Also, the children played no role outside of their own arc before and by keeping them away in that other country it doesn't seem that they will play a role in the future either. So what was the point in the first place?
Don't misunderstand me, I liked the children's arc, I'm a sucker for interesting filler and slice of life episodes in such shows. But it's really no good idea to end a show on them.
Also, the examples you used with LOTR and Star Wars don't really fit. Because
1. an epilogue between two seasons doesn't make sense.
2. Here it was 1/6 of the whole show. The epilogue in LOTR and SW is only 1/18 to 1/20 of the story, if you are generous.
Like I said, it's not that I disliked that arc, but the execution was just botched. Also sorry for the wall of text, you brought up so many points worth commenting on and I didn't want to just ignore them to keep it brief.