Hi there - I help take care of my best friend who has advanced RSD. She recently & very suddently started having the same type of hand spasms your daughter has. There is a danger that her hand will stay locked in that position if you don't pull her fingers apart, I'm sorry to say.
So I agree with Frogga - you have to undo the spasm yourself, right away. I have tried everything on my friend, and have found a method that works, with my friend: massage. I use 2 types of massage strokes, one is long gentle strokes down the length of the outside surface of the hand, starting above the wrist bone all the way to the tip of the fingers. My thumb is on the middle of her palm. The idea is to warm and relax the muscles (mostly warm them and calm the person). I also verbally inform the hand muscles that 'they do not need to do this!'
The second massage technique is to gently search for muscle knots, and do a firmer circular massage in just that one spot. The muscle 'knots' are certain places that hold the most tension. I'm not a professional so I'm not quite sure how to describe this - I just imagine where I hurt the most when I'm in pain (I have fibromyalgia). Good spots to try for release are the base of the thumb muscles, and about two inches above the wrist bone, up the arm. If you google "pressure points massage" you might find a diagram of the body that illustrates these.
This massage can take a reeeeeeeeally long time before the hand lets go. Sometimes an hour or more. The basic idea is to generate an aura of calm reassurance that you are certain the hand can, and will, let go. Relaxation is key, both yours and hers. She must be very frightened, and your own hands' emotional messages can fill her hands with calm healing.
As soon as her hand lets go., or even a finger, or even just a knuckle!.. slide your hand inside it! Not as if you are holding hands, but as if she were molding her hand on top of yours. If you can't get your whole hand in, even just a finger is good. Anything to open up some space. Grit your teeth... her clamping hand is probably gonna hurt yours awful lot! [not as bad as childbirth did though] Having a tennis ball or smaller ball handy would be a good idea too...slip the ball inside her fist as soon as you can get it in.
In sum: don't wait till you get into a doctor's office. You can probably coax the hand open, or pry it open, with some patience and mom-type gentle firmness.
Good luck!
M