there was me hoping for some decent support from other mums out there but instead being battered with a barrage of criticism over my child's eating habits which I am seeking advice from his health visitor! Definitely given me a disliking for these chat forums I thought we were all supposed to support each other as mums not make one feel much much worse than they already do!
The thing is that many of us are not just "mums," but also MILs, SILs, friends, sisters, DILs, (and all the male equivalents), and as such we host other people, too!
They may have been wrong to not have child-proofed their house before your two arrived, after having been cooped up in the car, but it does sound as though they made an effort with the lamb roast. Also, you said yourself that the Cheerios incident came after both you and your DS refused to eat supper, and after you refused to eat Cheerios, so it's not as though there wasn't any provocation before their remark. They must have been wondering how to get through all the remaining meals of your visit!
If guests are going to be limited in their diet, for whatever reason, they have a responsibility to manage expectations ahead of time, and to mitigate the effects of it, maybe even conceal the full extent of their limited diet by taking the hosts out to dinner, cooking a meal "to say thank you," etc.
There are many ways of dealing with the effects of intolerances, allergies, religious prohibitions and simple fussy eating. Springing these things on your hosts without warning, apology or mitigation is not the way to deal with it. Sorry. YABU.