Have a kids bike that already had 4 to 5 mm difference in chainline. Adding a 13t sprocket to the rear, which won't be "offset" [ in the proper, original direction]... so the chainline will now be 6 to 7 mm from being perfectly straight.

Both sprockets are 1/8" width, but I'm willing to use a grinder on both of them if a 3/32" chain would be best.

To pre-emptively answer some questions...

No, I can't move the front sprocket any farther inward. And I'm fairly certain that I can't move the rear sprocket any farther outward without going back to the original 18t, which I really don't want.

Would a single speed 3/32" chain be better than a multi-speed 3/32" chain? Or should I just stick with the 1/8" chain until i wear it to shreds?