I found the hide-and-seek aspect a bit difficult in The Lost Cousins by B.B Cronin, therefore, this is more for the older child (5 to 8 or even older). However, it is suggested for slightly younger aged group. It will depend on the actual child and if there is adult help as well.
Just announced as part of the NYT’s Best Illustrated 
Books of the Year 2019, it is described: “All great picture books make you see the world in a new way, but B.B. Cronin’s “The Lost Cousins” really pushes the envelope. It’s a tour de force work of psychedelia for kids, playfully dense and almost radical in its juxtapositions of eye-popping fluorescent colors. Of course, seeing is very much the subject of this book, the fourth in Cronin’s Seek & Find series.”— Bruce Handy
All I can say is, yes, it is psychedelia (yellow polar bears with Shriner fez anyone?) The cousins look nothing alike, but all some have similar qualities (some type of bear or cat or dog element. Basically, some type of animal). The colors are overpoweringly bright. Bright, in fact is an understatement. You might say sugary sweet, but this is almost vomit inducing in some ways. It is something your Scandinavian grandparent probably read in the 1920s or 1940s. The retro aspect could take you out of the scene quickly.
The story does allow some breathing room. The illustrations will be either something you love or hate. And do not forget your shades when viewing a page.