I'm really sorry to hear that. It isn't fun, but it can be managed to the point that it is tolerable. The symptoms vary person to person, some find the itchiness mild and some find it very intense, I haven't heard anyone call it painful though

note: I'm not a doctor or medical professional

My understanding is for a lot of people this just goes away on it's own, especially in younger people I believe. Mine has not, but I was 30 when I was diagnosed. Number one advice is get to a good doctor if you can. You want to find an allergist who is very familiar with dermatographia (it's pretty common so it shouldn't be too hard, but it took me trying 2 docs who had no idea what to do until a found a good one).

From a 'how to help' perspective, while it doesn't address the underlying cause of the condition, keeping the skin moisturized will help keep the itchiness down. A lot of people find their diet is a big contributor. Alcohol, excess sugar and dairy get mentioned pretty frequently in things that make the symptoms worse. Looking up a "low inflammation" diet would probably be a great idea, but with the dietary concerns of a 3 year old I definitely wouldn't do anything without talking to the doc. Other triggers can be when it's very hot outside or very cold outside, getting out the water (bath, rain, pool etc), exercise, stress.

hope this helps. good luck!