How to make the end of a kaleidoscope?
What about those small plastic clear round takeaway restaurant containers?



Image from AliExpress


They are clear.
Have a lid (also clear)
Are wedge shaped (making it easier to insert into the end of the tube)
With the lid accessed from the outside you could then change what's in the kaleidoscope. 


Or you could pre-make different containers with different confetti and glue the lid down. The kids could then swap different containers into the tubes as they want.