How to do Ctrl+Alt+Del via Logitech iPad keyboard
If it's Parallels Mobile, you need to use the on-screen keyboard for complex keyboard commands such as this one. According to the documentation, once you bring up the on-screen keyboard, there are arrow keys on the top of the keyboard to switch to a "function" keyboard; from there, you can press "delete" with both the Ctrl and Alt modifiers active.

The Mobile Guide (link) produced by Parallels for this application can help guide you further. (the Page 19 area references the items I mention above)