First, it would be dreadful if an Enterprise class ship ever entered a conflict with an Imperial Star Destroyer!
Here's some helpful info:
ISD:
http://en.wikipedia.org/wiki/Star_Destroyer
Enterprise:
http://en.wikipedia.org/wiki/Starship_Enterprise
IMO, the ISD is in a different class -both literally and figuratively- than the Enterprise ships and would most likely win the battle. It's simply bigger and badder and also carries 72 TIE fighters.
Enterprise has a tachyon beam, which I don't think the ISD have, and some have cloaking abilities, which I think came from the Romulans.
Of course, I would create a story where the Enterprise won as ST is a sentimental favorite of mine.
I would tell your kids about each ship's tech and let them be imaginative about which would blow the ship out of the other.
Best,
Kurt