The best way to develop your ear is in my experience memorizing intervals.
It's a very simple task.
You just write down on a piece of paper one interval (or some mnemonic melody added to it)
Then you sing it to them several time.
Then you ask them to do the same thing everyday at home.
Sing that interval starting from every note and using the sound TA-TA.
They don't need more than 5 minutes of practicing every day.
I would focus on each interval for a week or more.
After 2-4 months they will know the sound of every interval and will recognize the intervals when they ear it. Such subconscious knowledge has an extraordinaty influence ear development.
So let's say you start with a 2nd major or a tone.
The best way is to teach tone and semitone together because when they are sung sequentially it's easy to feel the strong difference in quality between the two:

Let's say you want to teach the 3rd major:
For the 3rd major is better to reach it through singing a scale of tone.
And after having encompassed the interval through the scale singing the interval alone.

It's really an easy work.
Sing it for them. Just one interval at a time.
Tell them to reproduce the aural relationships of the interval starting from other note.
For example play a C and then a D consecutively and many times.
Then play a F and ask "can you sing me a sound that will make the two notes sound similar in sound to the notes I played before". At this point they will sing a G (even if they don't know you played an F and they singing a G)
Then tell them to do this work everyday at their home.
First they sing the interval you're studying and then they must go they piano and sing the interval starting from the same note. They need no more than 5 minutes everyday focusing on just one interval for 1 or 2 weeks. No more than one at a time.
The 4th perfect can be taught as a "conclusive" sound.
It can be memorize in a matter of 1 week by focusing on this "end point" quality of this interval. Like the Ta-Ta of a circus number.
The 5th sound like a question:
C - G?
and can be memorized by focusing on this "sospension" quality of its
Funny that its opposite (G - C!) sounds like the answer to the question
and so on ...