You are right, I have tried again and have now completed the game. The problem is the 250$ limit, because you do not know that you will get money, or how many, for bringing back the children. So if you have too many money this will be a problem. The most logic way to end the game (when you are able to deliver the children) will be with the boss fight in the mine and then the respawning boss will not be a problem, but if you have not delivered all children the boss will keep respawning if you leave the screen and come back. This does not happen with the bird boss. A great game. Cheers!
Another bug I have found is that if you use the Mothwing Cloak to fly directly into a wall, you will hang in the wall at the place you hit it. You can get free at any time so there is no problem in it, but if you jump upwards from where you hit the wall, and use the cloak to fly into the wall again you can actually get access to the whole screen (places you were not meant to be) on every screen. For example you can use this trick to get out of the right side of the Dirthmouth screen (the screen with the old man, the well and the first bench), but as this was not intended, and there is nothing to the right, you will just re-enter the screen again from the left side of the screen ;-)