Depends if the big kid is male or female. The flex issue would be about right as a big kid would weigh about the same as a small female (unless the big kid is Sam from the biggest loser) but a male tends to be stronger in the upper body and uses this to help turn while a females centre of gravity is a bit lower and can sit a bit further back on the ski. That's why a female ski has the binding marks a bit further forward than a male ski.

Basically, if you're not a pro then these issues can be a bit of a moot point. A few cms shorter is good, because it might be a bit slower, but it'll be a bit easier to turn.

Enjoy the season. I'm hoping to get up to the snow for the first time in years.