I did a little searching Jingles. I'm not sure how active this is but there is a group for pediatrics
/www.rareconnect.org/en/community/mastocytosis-and-mast-cell-activation-disordersThere are a lot of support groups on Facebook. Some are listed under Mast Cell Activation Syndrome and some are under Mast Cell Activation Disorder.
Sending positive and healing thoughts to you and your little one.