I've not done the mother/daughter spa package at the Grand Floridian but I have had various spa treatments there and my daughter has had the 'my first spa experience package' (facial, manicure, pedicure were the components of her package). IMO, the service and treatments are quite good and 'worth' the money, but I find that the GF spa staff make a harder sales push for the products used during a treatment than does the staff at Saratoga Springs.
The same, or quite similar, treatment packages are offered at Saratoga Springs, and I prefer the Saratoga Springs spa over the Grand Floridian. My favorite spa package at Saratoga Springs is the Mystical Forest Retreat and I do think it is a very good value for the price. My daughther was 13 last year when she had the spa package. The year before when she was 12 she had the 'first' facial and manicure. She enjoyed both experiences. I suggest checking to find out if there is an age limit (e.g., does the child need to be 10 or older). When my daughter was under 12, she would take part in one of the GF's children's activities (e.g., Pirate Cruise) and I would go for a spa treatment.
Good luck with your decision.