On my CRF I use Silkolen 5w Fork oil, as far a how much look in your owners manual, or call your local shop. They can give you a minimum, maximum, and stock. The minimum will make your forks soft and the maximum will make them firm. Hope this helps.
I'd worry more about changing it out often and the height of the oil.
If the kid is jumping well, you may want to have the oil height a bit higher than stock. Experiment with it at 10cc increases.
You can use ATF with no problems.
They don't publish a Clymer for this bike, you might get a shop manual off Ebay like I did. The standard oil height is 90mm, minimum 120 and maximum 80.
Depends if you're a half full or half empty type.
The number is the distance from the top of the oil to the end of the tube. So 120mm would be the minimum amount of oil and 80 would be max.
Anyway thats how the yamaha manual states it.