Without a lot more information it's hard to say. Your description fits with injestion of something your child is allergic to. In allergic reactions we worry about breathing problems, and we watch for the speed of onset of symptoms. If this rash took a day to develop, you're probably fine. Check with your doc by all means, but it's likely you can treat this at home.
If however the rash develops over 15 muinutes, consider at that time transporting to an ER or calling 911.
In these questions, the more data you can provide, the better they can be answered. Feel free to use the discussion area or even the answer section to give as much info as you can.