Fischer makes a true SL at 125, Head makes their iSl-RD team starting at 126, Nordica/Blizzard starts their SL at 122' and Rossi/Dynastar has their SL at 126 I believe as well. Note all the above are true race construction (titianal, sintered bases, etc.)
I'd have to say that unless your 8 year old is big, strong and/or really cranks on the turns, you'd be better going with a multi event ski over true SL as they are not going to be bending the SL properly. Fischer makes a great 120 (RC4 Superior.....not the race) but it's also pretty beefy.
Head makes a really great i-race series that has some race construction but super forgiving for kids that are learning.
Everything else that doesn't have metal in it is pretty much the same.
For perspective, the only 8 year olds we have on real SL skis with metal are the ones that rail their turns, create very nice angles and subsequtly almost always end up at the top of the podium. E.g. They are few.
Only you and their coach can tell if they are ready for a big kid ski.....you do not want to over equip them.
My 7 year old is 52" and 72 lbs. Currently skis the Fischer RC4 Superior and is a very strong skier/racer. (Has won every GS race by over 2-3s this year so far.....regularly beating all but the very fastest 8 year olds (both genders). Free skis double blacks, will drop 10' cliffs and hits up to the M sized kickers in the park. Next year (8) she will have a 125 SL under her boots.