Time to go on a rant!
Heck ticks me off if used in a phrase such as "what the heck?". If you don't default to hell, then I hope your spleen explodes. I mean, aside from the overly-religious, why do you have to stick to heck? Hell is not a terrible word, it isn't even bad. Let your kid say it. Kids can curse, it's fine. They're only words. It's bad if they use words to hurt others, but if my 9 year old stubs their toe and curses, I'd laugh then remove their shoe to check, no harm done. Big difference between the two.
People always have and will continue to baby their kids this way, and it's annoying. Anyway, my opinion.