Name of the bias where someone really needs something after they find out it exists
It's not a bias. It is natural human nature. At least the 2 year old's case is.

It's just like how you would not have thought of going to Six Flags (An amusement park) unless it was mentioned to you. 

When the 2 year old hears ice cream, the kid thinks of the sweet taste, or the pleasure ice cream brings. In the kid's case, the case is impatience or inability to defer gratification. 

For the employee, it is simply that he thinks the cycling of the tank and fish delivery would take less longer than a week or two.

The two examples are both different things. I am positive that the 2 year old's case is NOT a bias. The employee's case may simply be surprise.