Last year the younger one skied on Atomic Race 10 120cm and the older one on the Rossi RSX Racing Jr (with riser plate) in a 130cm.
My understanding is that the riser doesn't affect stiffness (not sure) but it certainly adds weight. I have been told by some that both my girls should be on risers and I should also be concious that they have skis with a sidewall and slalom radius.
The thought was that a longer ski provides stability and with a true slalom radius they will get plenty of turnability (is that a word?). The suggestion was to use the one daughter's 130 Rossis for the younger one and to get a 140cm junior slalom racing ski for the older one with a plate.
At 66 lbs I was leaning more to a lighter 130 junior racing ski Rossi RSX (without the plate), Dynastar Omeglass, Atomic Race 10 or Racetiger Jr all without riser plates for the 8 yr old.
For the 80 lb older daughter I was thinking about a similar ski to what she had last year in either a 135 or 140.
I have seen the comments regarding a ski that challenges for further development without overwelming and hurting results and confidence. We ski about 60 days a year, most of which is race training or racing for the girls.