RE: Reason for recommended sail ranges?
The sailrange is an indicator of what sailsizes the board can carry for an avarage, adult, surfer. So you needn't look at the range and consider your little girl to fit it. The sailrange is also an indicator of what windrange the board can handle. The more wind the less controle you have over your board, so if the minimum sail is e.g 5.0 mé, you can always go lower, but you need to bare in mind that at a certain point you'll have too muych wind for the board and it goes downwind without you having any controle over it, which can be very dangerous.
If you are looking for a board of about 120 L and a big sailrange, i suggest you look at the SB carve. The 122 has a recommended range of 5.1-8.0, so it should easily be compatible with the 4.8-7.0 sails you have and your daughter can still go on it with her bigger sails if she's learning. Once she gets the hang of it and goes for bigger sails she should still be able to use the board.
Another option could be the S-type 115, but you may find it much harder to ride then a carve. However, the s-types sailrange is much more like your current sailrange.
Hope this helps
Haiko, AKA crazychemical