I'll try and answer this as best I can. There is no specific height the straddle jump must be. The amplitude deduction on floor is: Insufficient height (hip rise) on leaps and jumps Up to 0.20
So, we are looking for the kids hips to actually rise during the jump. If they don't, we have an up to .2 deduction we can take.
The straddle jump at L6 does have a minimum requirement for leg seperation of 150 degrees. The two deductions that are specific to the straddle jump for L6 (as listed in the JO Compulsary book) are:
Insufficient leg separation (less than 150°) Up to 0.20
Failure to land with feet together 0.10
As for how to get the straddle jump higher, I always tell kids to push out of their whole foot when they jump...so their toes should be the last thing to leave the floor. However, the coach should be able to address whatever specific problems your dd is having.