Welcome to the forum, Mandy.
At least you have an appointment. That's a start. But there could be something besides mold, that wasn't tested for, contributing to the hives. Scratch tests, the only ones that are pretty accurate, only test for a few of the at least thousands of allergens.
Start keeping a daily diary of everything the child eats, drinks, and medicines. Write down the time and any reactions that occur all day. Also include where the child is - inside, outside, at another house ... For more information on keeping your log do a search, in the search box at the top of the page, for "food journal" or food diary or food log. It may help your allergist.
I'd also start getting rid of mold or other allergens. Hopefully where you are you can open windows and air out the house. Then I'd get rid of carpets, rugs, musty smelling blankets. Wash all bedding and clothes, in hot water if possible. Then start cleaning walls, floors, closets, and especially bathrooms. Don't use chemicals any more than necessary because they are allergens too! I use a little vinegar in water for most cleaning. I have a steam mop for hard floors where I can't use vinegar. If you find mold you can often get rid of it with bleach unless it's gotten into drywall. It's not impossible to remove drywall and replace it, if that's where the problem is coming from. If you're using bleach, take precautions! I have the bathroom exhaust fan going when I'm cleaning there and I only use a little at a time..