We have recently added our first rhythm exercise:

How to create rhythms exercises

The student is given a time signature, and a symbol (note or rest), and has to find how many beats does the symbol lasts.

The time signature can also use alternative display:

And we also cover compound time signatures:

I hope those exercises will help you with the teaching of durations.
We will soon work on more exercises on this topic, such as beat counting and bar-lines!