Families of children with arthritis are highly interested in the benefits of diet to improve their child's disease and future health outcomes. Previous research shows that the germs - bacteria and other organisms - that live in the intestines (gut microbiome) are important to how well immune systems work, and that what people eat changes their gut microbiome. The investigators want to study whether a certain diet - based on the principles of the Mediterranean Diet - will improve arthritis for children and whether it was changes in the microbiome that led to improvement. Fifty-four participants in this study will change their diet for an 8-week period, and will have the option of remaining on the diet for an additional 4 weeks. At three time points during the study (beginning, 8 weeks, and 12 weeks), participants will provide stool and blood samples, will complete questionnaires about diet and other aspects of lifestyle and health, and will complete a disease assessment by a clinician. From collecting all these samples and information, the investigators will be able to determine if the diet was successful in improving disease activity in children with arthritis and if the gut microbiome was changed as well. This study will help the investigators figure out if a larger, and more definitive, study like this is possible to do in children with arthritis and will help the investigators design a bigger multinational study to confirm how diet affects disease outcomes and the microbiome in children with arthritis. If successful, this research will provide scientific knowledge to help families make their way through this difficult to- navigate topic.