Plugging your nose does not, as you put it, "Cancel out all taste". It simply eliminates one level of tasting and in the case of Jelly beans that level isn't all that important. Think about it, how often do you actually smell jelly beans before eating them vs something like chocolate or in a non-candy sense, chicken? If mom had tried this test with Cheese or the like she would have had a better shot proving her daughter wrong because for that the extra layer goes a long way in letting you know what your eating. Seriously imagine trying to tell the difference between Dolcelatte and Roquefort without being able to smell anything. That would be a vastly different and much harder test than "Is this a Pina Colada jelly bean or Cherry". TL/DR- If plugging your nose actually cancelled out all taste, we wouldn't have people do it because tasting no flavor defeats the purpose of trying to identify what flavor it is you are tasting.

Argumentatively, if the child is a Super Taster and the parent really sucks at cooking, isn't that kind of just constantly torturing the kid to force them to eat what the parent cooked? Notice the OP specifically says that the daughter has been using the excuse to "not eat my cooking". Which implies that she is in fact eating something. That could mean that the actual issue here is simply that the parent in question is a terrible cook who should either take a cooking class or start allowing their kid to prepare their own meals.

I'm not sure what you mean here. The OP did the right thing in blindfolding her child and plugging her nose for the taste test. The only thing she did wrong was choose jelly beans which 1. Are extremely child friendly and therefore something she is likely to be used to and 2. Very light on scent so they don't lose much by eliminating it. If she had wanted to do an accurate test she would have gone with something like cheese which actually does rely heavily on both it's scent and actual taste in order to create it's flavors.