I think the last line was out of line.
If you came up to the child politely and explained the physics, then smiled sweetly and left, you'd be a bit of a hero.
(After all, the family *was* visiting an observatory. Clearly they are not anti-science.)
The father gave *his* child a beautiful, poetic explanation. Your explanation would have enhanced it.
Instead, you found it necessary to toss in that "nothing to do with G-d" line, which was hostile and arrogant.
You purport to care about the intellectual life of this child stranger, but you were really working out some issue with your own upbringing.
Had you been polite and left off that last sentence, the father likely would have thanked you.
He could even have saved face afterward and said "...and that's the way G-d did it..." and your physics lesson would have been transmitted intact.
Instead of stars, what will this kid remember? A jerk who invaded the intimacy of his time with his father, making some point about gas.
You say you care about this boy having access to science, but all you accomplished was to undermine your credibility.
For all we know, the father didn't know the scientific answers himself, and was doing his best. Perhaps, had you been polite, both father and son could have benefited from your knowledge, rather than resenting your attitude.