Hai to Gensou no Grimgar, Kujira no Kora wa Sajou ni Utau Recommendations
Stylistically very similar. Depicts dark themes and stories with a delicate and light colour palette and whimsical style, the contrast works very well in both. Light twinkly music.
Storywise Grimgar is very much rooted in JRPG traditions whereas Whale Children has a more unique mythology and setting.
Both series have an element of beauty and a hint of sadness within the story. The animation in both series are very similar in a breathtaking sense. The soundtrack is amazing so far as well. Both have children protagonist that set out to explore their worlds and although they seem innocent, there seems to be a underlying plot that is in the works.