|Author||Detective story||Children's story|
|Joan Aiken||Trouble with Product X (1966)||Arabel's Raven (1972)|
|Nina Bawden||The Odd Flamingo (1954)||Carrie's War (1973)|
|Nicholas Blake (Cecil Day-Lewis)||A Question of Proof (1935)||The Otterbury Incident (1948)|
|Michael Bond||Monsieur Pamplemousse (1983)||A Bear Called Paddington (1958)|
|Christianna Brand||Heads You Lose (1941)||Nurse Matilda (1964)|
|Peter Dickinson||The Glass-Sided Ants' Nest (1968)||The Weathermonger (1968)|
|A. A. Milne||The Red House Mystery (1922)||Winnie the Pooh (1926)|
|J. K. Rowling (Robert Galbraith)||The Cuckoo's Calling (2013)||Harry Potter and the Philosopher's Stone (1997)|
|Jill Paton Walsh||The Wyndham Case (1993)||The Emperor's Winding Sheet (1974)|
|T. H. White||Darkness at Pemberley (1932)||Mistress Masham's Repose (1946)|
And what does Japan look like? I don't really know; but my impression is that writers in both genres are rare (or were until recently). One exception is detective story juveniles, which seem to be quite common in Japan. Many authors of normal detective stories seem to write these too. Both EDOGAWA Rampo (江戸川乱歩) and YOKOMIZO Seishi (横溝正史) wrote stories with their series detective aimed at children. That might have the advantage, I guess, that the authors are not just selling a children's book, they're preparing readers of their adult books. Of course it might work the other way: people who read a KINDAICHI Kousuke novel as children might associate the books with childish reading and avoid them as adults. Off the top of my head, I can only think of Tolkien who wrote books with the same characters for children and adults in English.
Apart from juvenile detective fiction, I only know of NIKI Etsuko (仁木悦子), who as well as detective stories, wrote a successful collection of children's stories under the name 大井三重子 (OOI Mieko), 水曜日のクルト (Suiyoubi no curuto, 1961), and MIYABE Miyuki (宮部 みゆき), one of the most successful modern Japanese crime fiction writers. Several of her mysteries have been translated into English, e. g. 火車 (Kasha, All She Was Worth, 1992). So have some of her children's fantasy books, e. g. ブレイブ・ストーリー (Brave Story, 2003). (I haven't read any of her books yet; but I've got a couple in my pile of books waiting to be read, so I'm sure I'll get around to her soon.) There must be more than that, I imagine. What have I missed?