my administrator password. How can I access to my computer??

Do you have the original disks that came with the computer?

What model iMac, and what OS?

I suggest you set up a second administrator id that you do not give out the password to your kids. You should be needing to give out the password to the kids anyway.

Change Password Mac OS X: Changing or resetting an account password via GUI: Resetting a user's password Resetting the original administrator account password

Here is how to change the password from the install dvd/cd

How to run reset password from your startup DVD.

1. Insert your startup DVD into your reader. Power down your machine. Hold down to the c key. Power on your machine. This will bootup your startup DVD.
2. This will bring you to a panel asking you for your language. Pick your language.

3. 

4. 
   

5. You you come to the Install Mac OS panel. Do not install.
6. Click on Utilities menu item. This will give you a pulldown list of utilities.
7. Click on Reset Password.