If you have root, I have created an app, AppShare, that will do what you want.
Paid apps will most likely require you to add the Google account that you purchased the app with to the secondary ('Callum') account or else they will probably fail the licence check. This may vary from app to app though.
You will have no problems with free apps though, and you should even be able to disable system apps such as Gmail etc. to prevent kids messing with anything you don't want them to touch!
Without root, I think the only way to do this is add your Play account on the secondary account and add some sort of PIN protection.
Although this means that your emails could be accessed on the second account, unless you then remove it after installing the desired app (which will be a pain everytime!).