She needs to simply say to her friend, "I'm sorry--we can't invite the kids. We've already taken several family members and friends off, and the backyard is just too small. I only committed to a 'plus one,' so you'll have to tell the boys."
Ten to one the boys aren't THAT excited. Maybe a little bit, because kids often don't get invited to weddings anymore--it's much more common to not invite them.
She shouldn't worry about this, and simply matter-of-factly convey the *facts*.