Most likely it is. Fiction books often take us to imaginative worlds and 'Life on Mars' could be one such creation, presenting fictional characters and storylines.
One great 'trip to mars' science fiction book is 'The Martian' by Andy Weir. It's a thrilling story about an astronaut stranded on Mars and his struggle to survive. The book is full of scientific details that make the story seem very plausible.
It usually has vivid descriptions of Mars' environment and technology needed for the journey. Also, it might have interesting characters facing various challenges on the trip.
A non-fiction book for kids is one that shares real stuff. It might teach them about space, how things work, or real-life adventures. It's not made-up stories but true and helpful information to make them smarter about the world.
Start by choosing a topic that kids will find interesting and relevant. Make the language simple and engaging. Use lots of pictures and examples to make it fun and easy to understand.