OK, 31 kgs. (68.3 lbs.)..... so very light in weight.
I would let her try the 115 liter board she has become accustomed to, bt remove the
center fin and help her to learn to stay upwind by railing the board some.
As soon as she can stay upwind on the 115 liter, let her have a go on the Kode 86 and
You may find that at 31 kgs she can get the smaller boards planing a bit easier.
Sounds like she if getting good instruction and making terrific progress.
At her weight, planing on a 3.0 is quite possible in about 12-14 knots of wind.
Let her work on the footstraps at her own speed.
She may suprise you and just get into them very easily, but she may also be a little afraid
of them. Best to let her work out the "fear factor" here.
Encourage her, but avoid pushing her.
If the school has a big Start, put some footstraps on it and go out with you sitting.
Put the little rig on and let her try the footstraps in lite winds with you onboard to
ensure everything goes well.
Hope this helps,