Hi, Linda!
Good question! And I imagine you will find as many answers as there are people here, and probably none will be exactly right because God and creation are so big and mysterious we cannot wrap our human minds totally around it. And I do think that is an acceptable answer for a child [lots of things are mysterious to them and I imagine he will get that]. And I also think a great response for a kid's question is "What do you think?" -- you might be surprised by his insights and thoughts.
If you want to take it further, I think it is appropriate to cite the verse about a thousand years being like a day to God. 2 Peter 3:8 "But do not overlook this one fact, beloved, that with the Lord one day is as a thousand years, and a thousand years as one day." (ESV)
Time is relative.
Let us know how it goes. We will pray for wisdom and insight as you meet your students where they are!
Thank you for your support for so many years, and continued blessings on your church's rotation program.
Amy