A.L.
The now 13 yr old boy here has broken out in hives since he was 1st introduced to chocolate!
I have alwayz bathed him in cool water with oatmeal soap after a breakout (this chocolate thing was DIFFICULT being a kid to say the least) sprayed him with benadryl that I keep in the frig & gave him a benadryl pill just for kidz (in the beginning the Dr. wrote a prescription for liquid benadryl that had dosing amounts according to weight). Hives generally are gone in the same length of time that it took to show up. Try to keep clean, cotton clothing on, the less the better. Extreme heat will make the child miserable. Extreme playing will make them worse as well so keep them inside doing building blocks, etc. VS going outside & rough housing with other kidletz or the family dog!
Good luck!