If you want to give them a shake, I left home at 15 and have been paying my own way ever since. I bought my own house at 21 (mortgaged) and regret nothing. Maybe sit them down and work out a budget. See what they 'need' per month, set an amount for savings (house, cars etc) and you can fairly ask for a set amount.
Hope this makes sense!
Trying to suggest a way so they know that you are being fair and have put their needs first
They don't know how lucky they are!
You're definately NOT a Bad Dad!