I was excited to read this thread, as I am dealing with this very question right now. However I was laughing as I read it because the challenge I am running into - i.e. that my son's coaches have a range of opinions and reasons for those opinions when it comes to skiing SL, GS or multi ski in U10 - are practically verbatim the same as in here!
When he was U8, my son was on a multi ski, and that worked fine for him. However once he moved up to U10 last year, it was recommended to us that he move to an SL. In large part, and I still 100% agree with this approach for last year, the goal was to challenge him by moving him from a softer easier to bend multi (Blizzard) and onto a more genuine race ski (Head iSL RD) that was stiffer. I think it achieved the goal because the first two weeks he got spanked by them because it was less forgiving of occasional bad habits, and it caused him to up his game. By the end of the year his turns had improved immensely, and those iSLs were instead rewarding his improved technique by really holding edge well at high speed. Based on the NASTAR-style time trials he is one of the better racers in that U10 group (gold level time, and about 2 seconds off of platinum time).
SL ski if only one ski. Better skiers should also spend time on a GS ski (not for specilization) but instead to properly learn to carve the ski rather than just ride the sidecut.
Fundementals are key, GS ski illuminates things that can be masked with SL ski at that age.
I just saw this on the Squaw Valley U10 Ski Team Website Program Guidelines section:
Skis: 1 pair of GS or multi-discipline race skis regularly tuned
At the end of the year I was talking to his coaches and asking about what we should put him on for next season (which will be his second in U10) so as to best suit his level and to support his development. The opinions I got were basically similar to the quotes above.
One coach said that he definitely should stay on the iSL and just increase the length. One coach said that he should go to a GS ski but shorten the length so that he can't just rely on the sidecut to help him and to challenge him to try and turn the GS ski without the SL sidecut helping him (and in the shorter length the GS radius isn't tragic anyway). And then a third coach said to get a multi race ski and split the difference (although at least one other coach worried that many multi race skis would be too soft for him next year).
So... there I am still stuck with my dilemma. haha
I am leaning towards staying with a iSL and just moving the length up, and certainly my son himself said he loved the iSL and would happily ski it again!