So I want to lock my future MX-890 remote (have not gotten it yet) so the kids cannot use the system if I don't want them to.  So I have created a 'Remote Lock' page, where if you press the button on the screen it will required you to enter a password.  For this I am using the IF/ELSE commands and I am wondering if this will work as I am intending.

I have also made the MAIN button jump to the same lock screen and I have removed any punch through commands from the power buttons.  Obviously, the kids can circumvent all of this by simply rebooting the remote by removing the battery, but I hoping they will not figure this out; they are 10 & 6.

I don't want the remote locked all the time, only when I push the Remote Lock macro.  So will this work?

Looking at the screenshot you posted I don't think that's going to work. Go to this linkhttp://files.remotecentral.com/urcg/58-2/index.html. There is a file called vision 21's pin code. The file is for a mx-3000 (so you would need the editor to view the file) but the concept should carry over to the 890. Also if you are having the main button jump to the lock screen just make that screen main page 1 and it will return to that screen after reboots or battery pulls.

Braden Napier used to have a Youtube tutorial on how to do exactly that with a MX980. If he's watching this thread he might post a link to it. I didn't see it in the Youtube search.

But that would also mean that whenever you hit the main button in any activity it would go to the lock screen as well. Unless I changed the main button in each activity to go to Main page 2 and that's just too much work. :) I am not too worried about the battery pull for now, if it becomes an issue, I know how to fix.

Why not hide all the device pages so they are not accessible from the home screen, then put a hold and release macro on any button you want. So you can get to the real home screen by holding "8" for 3 seconds. All other buttons on the home screen do nothing.