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.
See https://www.goodreads.com/author/list/7438442.Chris_Ferrie for more info.