From someone who tries to avoid altering a pattern...
The length looks like it should be pretty easy to modify. Measure your daughter from knees (or however long she wants the coat) to wherever the waist is on the pattern - this may be a place where it decreases for a waist, or perhaps where the pattern changes for the chest part of the coat. Then when the pattern tells you to knit X inches then switch to the chest pattern, just knit the number of inches you'd like.
I have never been brave enough to change sleeves, though, because there are consistent increases throughout the length of the sleeve. You might try this, though: directions for increases usually say to increase every 5th or 6th (or something else). You might be able to increase every 4th or 5th row (that is, one row more often), but you'll want to be careful that your sleeves don't end up too short.
There's no guarantee that this would work as you hope. One of my sisters is a good enough knitter to design her own sweaters, but I'm not!