Okay, nothing to do with hypothesis vs assumption (excep that sam's right - they're not the same; a hypothesis is a theory based on information/facts, while an assumption, well, isn't - which is apparent in the thread based on what a lot of posters are saying despite the absence of supporting information).
Anyway, there are three possible explanations:
1) The kids knew they hit the windows.
2) The kids didn't know they hit the windows.
3) Somebody not of the OP's household hit the windows.
I realize the OP took responsibility - but there's a 1/3 chance she's actually not, and a 2/3 chance her kids didn't say anything because they didn't know - either because they weren't aware the BBs hit the windows, or because somebody else was shooting.
That's not statistically correct, because the probability of those three possibilities is not equally distributed along the statistical curve. In other words, without evidence to the contrary, we know that OP's sons shoot guns in OP's backyard but we do not have any evidence that any other child has ever done so, so the fact that it could have happened does not allocate it a 33.3% share of the odds. For example, if I list "An alien shot out the window with a laser" that doesn't mean there are now four possibilities, each with a 25% probability.
Now we can also likely increase the probability that the OP's children were acting carelessly and not tracking their shots, and therefore were unaware of their actions, but that is further evidence, in my opinion, of why they shouldn't be shooting a gun in what, by OP's comments, appears to be at least a semi-densely populated area. Guns are NOT toys and the second amendment doesn't allow people to shoot guns off willy-nilly in unsafe environments!
While it may not have been intentional, it also would not be something I'd ever call accidental, as to me that implies that it was the result of uncontrolled and unforeseeable events. At best, OP's sons were exorbitantly careless and,
IF OP knowingly allows such behavior, was irresponsible to the point of negligence for allowing them to shoot a gun in an environment that was unsafe. What if it had been a child, an adult or an pet that was hit? The fact that it was an easily fixable tangible piece of property is extremely fortunate, in my opinion.
Lastly, I'll agree with those who think $450 is very inexpensive to replace glass in three windows. We replaced a picture window a few months ago that cost over $1,500, due to its size and shape.