steam
Is it possible to disable Steam notifications (friend logs in, messages you, et cetera) in-game, and do so without disabling the community completely? That is to say I can still access Steam in-game – it just does not display the messages.
Emphasis that this is for in-game only, I don't want to just disable those notifications everywhere nor do I want to have to turn them on and off before and after I play a game.
Unfortunately, Valve will not let you transfer games to a different account, unless they are gift purchases. As suggested in SqlACID's answer, you can create a separate account login for your son that still uses your Steam account login and password.
If you have Windows Vista, Windows 7 or Mac OSX you can additionally setup parental controls to disallow the launching of games that you don't want him to play.
If you want him to be able to launch games directly from Steam, I would suggest creating a new game category specifically for him and then add all the games you want him to have access to. To do this, right click a game and select Set Category..., then add a name. You can then select that category at the top of the games list. Here's a rudimentary example:
Closing steam while downloading an application and resuming the download later without losing progress is a feature that is supported by steam. Steam will pause the download when closed and automatically restart it when opened again.
So to answer your question, this is not a known bug and it isn't being fixed by steam currently.
In order for steam to restart the download it needs to have access to the games Local Content. If these files aren't in the file system then it'll have to restart the download. Try navigating to your steam Library folder and see if these files exist. If they do, point steam to them.
If the files don't exist it's possible that your operating system handled them in an unexpected way. This could be the result of countless different variables though. Either way it may be worth submitting a bug request to steam if they aren't there.
Edit: I did some more research on this this morning and found a work around that may of use to you. If your Local Content for the game is available in your file system you should be able to force steam to restart the download where it left off last by copying the downloaded portion from your C:\steam\steamapps\downloading\ folder to your C:\steam\steamapps\common\ folder.
Best Answer
If you go in-game and do shift+tab, you get that odd overlay menu. in the friends pop-out there is your name, and a settings symbol next to it.
click it and you get a "friends light settings" menu
go to the Notification tab and uncheck everything