Here's a sprint/pace set I tried for my top-level age groupers (10-13). My thought was to get them kicking fast, making their legs tired and breathing hard, and then challenge them to try to hold 2nd 50 goal 100 pace at the end.
2 x (round 1=prime stroke, Rd 2=Fr)
K 3 x 25 FAST @ :35
1 x 50 swim @ 1'00
K 3 x 25 FAST @ :30
2 X 50 SWIM @ 1'00 (both faster than #1)
K 3 x 25 FAST @ :25
3 x 50 swim @ 1:00 (all 3 faster than 2 &3 above at around 1/2 goal 100)