in a book intended for children always presents a challenge. Someone told me recently that adults should not solve the problems posed in the plot. But, does this mean you really need to do away with the adults? There are some memorable adults in children's books.
Some of the adults that remain firmly in my mind are Bendy's father and Caxton in "The load of unicorn". They are both men ahead of their time but Bendy's father is the one who has really met the challenge of the new technology. Then there is Maria's guardian and her governess in "The Little White Horse", indeed a whole range of adult characters who are handled with such apparent ease that the book must surely have been very difficult to write! There is Mrs Wintle in Wintle's Wonders - ambitious, demanding and jealous. She really is "one of those mothers"! There are the Callendar parents in John Verney's books - slightly bohemian but still part of the real world! Elfrida Vipont's Lark in the Morn and Lark on the Wing have a range of very believable adults too. They are a steadying influence through the trials of growing up but have their own eccentricities.
I do not think adults need to be removed from books for children, even books for teenagers. In "real life" many ten and eleven year old children are under constant supervision. In Australia there are still any number of slightly older children who are taken to and from school each day and who would not be permitted to go out with friends except under strict conditions and controls. Anything else would be less than realistic for most children and trying to plot something without the presence of any adults may sometimes be unrealistic.
Adults can be part of the book - but yes, it is very difficult to make them believable without having them interfere in the plot!