Poor mite :( - my DD also has a fractured hand ATM, but she's older so is coping okay
& yes do give her the homeopathic arnica, it can't harm & may even help (even if the nay sayers say otherwise
You may also want to try her with Lavender & Roman Chamomile Aromatherapy oil, a drop or 2 on her pillow or clothes, or mixed with a carrier oil for a foot massage always works well - they are not pain killers as such, but are oils that are perfectly safe even for babies & help with relieving stress & help relaxation & sleep - if she's in pain, then her body is stressed, so anything stress relieving will help, as heightened stress magnifies pain - IYSWIM
Also - Epsom Salts is loaded with magnesium & adding a couple of cups of bath grade ES to a bath or if she can't manage that, a foot bath & soak for as long as possible - 20 min plus is best, it will get magnesium into her(proven in tests for the naysayers & keeping up her mag levels will help her body regulate & manage pain signals properly, so that she is more comfortable - you can add the aroma oils to the same water,
Also Black Cherry Concentrate can be bought in health shops, it's not cheap, but it lasts a while & has been scientifically proven to help reduce pain & makes a very pleasant drink if mixed with sparkling water - around here its was always known as "Magic Pop" - again safe for even one as small as yours - the cherries themselves work too, but the most potent ones aren't so easy to find & are probably more expensive than the juice
HTH