Once, it was thought that the universe didn't even exist. The very idea of a solar system was, at one time, not even accepted. Galileo had his head on the block just for showing the orbit of the Earth around the Sun. Theories of the universe have evolved immensely. What was once thought to be a limited universe is clearly not. In fact, physics now proves that the universe is expanding.
This is based on a number of factors and observations. It may sound like metaphysics in some ways. The interesting part about that is there are some similarities. As it turns out, those old tomes of metaphysics have demonstrated and even foretold the full understanding of the universe, bowling over the understandings as they were known.
Presently, the expanding universe theory is becoming a solid idea that is now clear and understandable. With certain principles, there is an understanding of the mechanisms involved with an expanding universe, including how it affects time and space in general. For example, there are topics such as Circlon Synchronicity, Gravitational Expansion, Absolute Motion, and Atomic Dynamics among others to consider.
The good news is that there are some excellent resources available to help make sense of all this. If you are a disbeliever for some reason, take a look at some of the information available in books and online. It is an interesting subject to think of and, your kids will be learning about it soon. When you do look at the information available, you may feel inundated at first.
The best advice to follow then is to break everything down in steps. Wrap your head around one aspect at a time. A good reference will guide your understanding so you will be able to understand what is being stated.