You could try - " All things are present to God at all times. With God it is always 'now'." Then introduce the word omnipresent.
Good suggestion! I will remember "omnipresent."
I use an explanation rather like that, but kids, especially boys 10-11 years old, really like discussion that sounds "scientific." They pay close attention when I bring up atomic physics, Richard Feynmann, and that sort of thing.
Also, in school they're generally taught a very primitive "scientific" materialism, and bringing in the complexities of advanced physics helps to expose them to a bigger picture.