Here is my take on it, if it was your idea and they invested in it they gave you a shot they intended to make money off of their money and your sweat, it did not work out and that is that you have no obligation to them. You both took a risk and it did not work out. That being said, they gave you the capital to take a shot and there are a lot of people that would be very grateful to get that shot.
So it is really contextual, where these guys scraping up their savings and the rent to give you the shot if so they took a bigger risk in investing in your idea than a guy that has several million lying around in the bank. To me that would affect my decision on the subject.
At another level it is kind of strange that they feel entitled to revenue from a new venture with new people who had no commitment to them. It is totally independent other than the fact that you are involved.
For me, if someone bet it all on me by scraping together what they could and I made it on a separate venture, I would try to restore them to where they where before they made an investment in me. Now that is just me, there is no right or wrong here. If it was a professional investor that knew the risk I would tell them to fly a kite. Finally, I would not give them stock if you are doing well and can spare the cash I would pay them back their investment and not a penny more. They have no entitlement to any gain that this company has produces and it actually robs someone that did take a chance on this go round of rewards. Because quite honestly, one of the others like a first employee is far more deserving of those shares than someone who took a chance on an independent venture.
So for me, if the investment was made by someone who took a big risk I would pay them back what they invested, no more. They are not entitled to it and should be extremely thankful for even that.