I grew up with my GPs and we are close, however they are rather old fashioned and don't see things as I do but I also know that I can be oversensitive and take things too seriously.
Yesterday we were there with some other family and DCs were running around the garden, we were sat by the big windows looking out. DS (8) was running around, came over to the windows and lifted his team shirt up laughing (no idea why). My DGD called him fatty in a joking way. My DN then came over and was pointing and laughing at his 'fat' belly. Everyone else was laughing. I said "it's ok DS, you're not fat" (he really isnt) and my DGD kind of scoffed and said I was ridiculous (or something long those lines, I can't remember the actual word) and of course DS wouldn't take it seriously. I said you never know and it can be how eating disorders start. I don't for one minute think DS will get an eating disorder from a comment like this but its not the first time they have said something like this.
They think they are joking but DS can be very sensitive and can take things badly (which I explained but I could tell they thought I was being stupid). I also remember silly little jokey comments from when I was young, they used to tell me I was oversensitive and that I take things too seriously but it's how I am.
AIBU to not want my children called fat even as a joke? Probably irrelevant as I know they won't listen anyway. They are lovely bow and we do have a great relationship but there are some things they just don't really consider to be an issue.