One difficult aspect of having young age group swimmers train butterfly is that their technique often deteriorates from beginning to end of a 25. It becomes what I heard one coach call “butter-struggle” instead of butterfly, a situation that is a recipe for bad habits to form. Below is a set that could be repeated over the course of a season to help swimmers maintain their technique while they gradually increase their fitness and the amount of fly they swim.
6 x 100 @ Interval that gives approximately 20-30 seconds rest
On #1 and #2 – the first 3 cycles off the wall are fly, then go easy free the rest of the length
On #3 and #4 – the first 4 cycles off the wall are fly, then go easy free
On #5 and #6 – the first 5 cycles off the wall are fly, then go easy free
1 x 50 fast fly from dive (to measure progress from one repetition of the set to the next)
A week or two later, the set could be adjusted to 4, 5, and 6 cycles. A week or two after that, the set could be adjusted to 5, 6, and 7 cycles, and so on. The interval could also be adjusted harder or easier, depending on how they seem to be adapting.
For even better results – include a required number of dolphin kicks off the wall (though I suggest you keep this constant so as not to give them too many numbers to keep in their head.
Another option — have the remainder of the 25 after the fly strokes be dolphin kick on the back instead of easy free to prevent sloppy freestyle from happening.