defaults write com.apple.dock contents-immutable -bool true osascript -e 'tell application "Dock" to quit'
Hide the iMac. Or spend some time with the child at the computer, showing them what is appropriate and what isn't.You could always just hide the dock with Command-Option-D.
Also, teach them Cmd-Z. It's a great timesaver/failsafe.pulling apps off? I sit down on the computer and often find 1 or 2 icons left on the dock,,,
Also works for nosy roommates, regardless of age.My 4 year old son has his own account set up that is locked down so that the only thing he can access is the Webkinz site via Safari.
He can't access or change anything else anywhere.
If it is just your 3 year old messing with your stuff when you are not around, assign one of the corners of your screen to activate a password protected screen saver that you can turn on when you leave.
When I see my toddler coming for the keyboard I just fire up Alpha Baby. He loves it, and it doesn't affect any of your settings. Hint: if you set up the preferences in AB to say the letters and shapes it'll blow your kid's mind!