Hello,
There are various types of arthritis like osteoarthritis,rheumatoid,psoriatic etc.Since you got them at pretty early age it would be good if you get it properly diagnosed by a rheumatologists and if already diagnosed then please mention it for proper treatment.
Till then active movements withing the pain limits and yoga that includes pranayam under a certified trainer will reduce the symptoms to some extend
Next Steps
get her
calcium levels checked and blood test done for any autoimmune disorder
Health Tips
Aquatic physiotherapy will be helpfull