This problem would make a good short challenge, for example first thing in the morning as you are taking the register. It requires some logical thinking and quickly leads to a generalisation.

Is there a way to make sure both 'slots' of the toaster are always being used?

Many children will be able to find a general rule for the minimum length of time taken for any number of slices of toast. Can they explain why the rule always works?

Using something to represent the slices of bread might help, for example, playing cards which are different on each side so that it is easier to keep track of what has been done.