I both write and illustrate. My first book, "Twinkle, Twinkle, Little Shark!" treated the animals as neutral apart from the pair of iguanas (putting on their pink and blue pyjamas), but the child in bed at the end definitely looked male, even though he and all the other characters were shown as silhouettes.
For my next book, Armadillo, I set out specifically to make the eponymous creature neutral. This led to some difficulties with the rhyming and scansion of some of the text, as I couldn't fall back on "he", "she", "his" or "hers". Even the candles on the birthday cake are pink and blue. It's up to the reader's imagination.
My "Bob" series, about "the best goldfish in the world" feature a male fish (he does wear a pink tutu in "Get Active, Bob!") but the books' narrator, Bob's young owner and best friend is an unspecified child. Even though I have two sons, I have a feeling that the narrator is a girl.
That just leaves my "animals playing musical instruments" alphabet book, "Narwhal Plays the Nyckelharpa". All the animals are gender-neutral with the exception of the stereotypically female "Kangaroo plays the keyboards", joey in pouch, playing a Hammond C3 organ with one paw and a Fender Rhodes electric piano with the other, like some marsupial Christine McVie...