Personally, unless your child is perhaps under 5 (maybe under 8), it could be a mistake to think of a “book for children.” I was given The Planets by Patrick Moore when I was ten. I still shelf it. Turn Left at Orion by Guy Consolmagno is an example of a book for learning to see the sky that will serve for decades as your child grows up. I trust that others here will have similar recommendations.
These may be too young for your kids, but <foo> For Babies is a wonderful series. I loved Bayesian Probability for Babies and General Relativity for Babies. Astrophysics for Babies is probably the closest in line with your request but many others are worth considering, including the GR title mentioned and There Was a Black Hole that Swallowed the Universe.