You're not dumb! The only really dumb question is one that you have and don't ask.
Adjusting your diet is not "simple" as you say, but you are likely to be very frustrating eliminating lots of things and waiting months to see if it makes a difference. That's a long time for your child to be miserable.
I doubt that your child is reacting to the calcium, so in that sense, adding it in won't hurt her. But it won't help you - see below.
Citrus means the fruits you mention, plus pomelos and other relatives. The apples, berries, etc. you mention are not citrus just because they have vitamin C. Just guessing that citrus is a culprit is problematic though.
I understand you want to replace nutrients from foods you are eliminating, but be aware that all vitamins and minerals work in concert, and each needs the others for full absorption and interaction. Taking a calcium supplement is not going to be effective, and neither is taking Vitamin C or any other individual or small combination (2, 3, 4, 10...) supplement. Most pills are not absorbed more than 15-30% anyway, plus they contain a lot of additives and fillers which prevent absorption and which can cause other problems. For example, they are coated with wax or shellac or other items so that they pop out of the mold during manufacture - not only is that stuff no good for you, it inhibits full digestion.
Comprehensive supplementation is a complicated science, and most of us are not walking around with PhDs - yet we try to read labels on vitamins and play "kitchen chemist" ourselves, mixing and matching. But we have no idea what we are doing - we either get insufficient nutrition, or we mega-dose, and both are really bad. So we spend a huge amount on over the counter supplements, either going for the bargain brands that we find in the supermarket or box club, or we spend top dollar at the "nutrition center" or the whole food place - and it's still not absorbed, it's not comprehensive, AND we have no idea where it's manufactured! Could be in any country, perhaps with poor standards, and just distributed/marketed here.
There are other food sources for the various vitamins you are mentioning, but the problem is, none of our foods have enough of these items in them anymore, and that's why we are nutritionally deprived and our children are also paying the price for that.
It is not "natural" to be food intolerant or "allergic" - and these are very new problems which never existed when most of us were kids. So there is a great deal wrong with the way we are eating and they way we are supplementing. Supplementation is essential (the AMA said so in a landmark study 10 years ago) so that means that virtually every adult and every child is missing nutrients - which is why we have epidemic rates of nutrition-related diseases and conditions.
There are solutions to this but they do not involved cutting a lot of nutrition out of your diet. It's about ADDING the right things in the right balance to restore your body to what it should be. Doing that piecemeal with this jar of minerals and that bottle of vitamins isn't going to work (even though it makes us feel better mentally) and it's generally very expensive for little result.
Working with top food scientists over the past 5 years, I've learned a lot about this and I hate to see your child miserable and you going crazy trying to solve it yourself.
Let me know if you want more info, but I really wouldn't spend the money on single ingredients. There's no science that says that's a good idea. There are commercials, ads and products, but no science.