And note that what you measure at home is not necessarily their marker....I've heard of kids that make a requirement on one ride but not another (with the same height requirement because the markers are a bit different).
I haven't been to US/IOA in a while, but we went to Six Flags this summer, and DD6 was a hair under the measuring stick (you could maybe have placed a credit card width between her and the stick) and they won't let them on.
It's for their safety, so I would never press it. Just don't let your kids get too excited about ones they may be close on because they may not be let on.
There will still be plenty for them to do - hope you have fun