In general, it is nice to avoid themes that are frequently used, because publishers are on the lookout to discard these. Young children often enjoy stories of animals, humorous happenings and family. Themes that center around school are good ideas for any age category. Older children require more original stories, ranging from problem to maturity to dating issues. Concept books teach a concept, perhaps science or math.
Informational books provide an engaging review of non-fictional elements.
Specialty books deal with creation of manufacturers and branding, like pop-up books and coloring books. Because these books are so small, one usually needs a stellar writing style to get them published. Writers like Shell Silverstein and Doctor Seuss have an intensely developed personal style. Their writing could be identified anywhere. Try and establish this same kind of indispensable style, and publishers will review your work more carefully.