Here's a set we did with our top Age Groupers (11-14) that worked really well:
(SCM)
4 rounds:
5x100 FR - 1:20 / 1:30 / 1:40
10x25 RACE - 0:40
- IM order per round, dive on Odds
Two things were important to make this set as good as it was for us:
1) The 100s were right at the kids' cruise interval (they had to work decently hard to get 2-3sec of rest only, nearly much playing wall tag the whole way). I would adjust those intervals likewise for a different group of kids or if we were swimming SCY.
2) Each kid had goal times for the 25s. Those were about 1sec faster than what they need to hold to make their goal 100s of each stroke this season, so they had to work pretty hard to make them.
It was a great way to get plenty of hard aerobic work, with a lot of fast swimming in the middle!