Not a big deal. Pot, especially in small quantities(which it would be in a single gummy), is relatively harmless.
First, not a safe assumption. I live in Oregon and am in Washington a lot. Gummies in Oregon are 5mg THC but 10mg in Washington. However, I have seen products which are
100mg in a single edible dose (not sure why they're sold, given Washington's laws, but it's not my industry). I saw one product which looked like those waxy kid's treats that have sugary liquid inside. 100mg. And in the same packaging as the same product in a 10mg dose. The only difference was 12-point lettering on the package...once it was out of the package, they were indistinguishable.
But yes, it was probably 5mg or 10mg. Relatively harmless compared to opiates or plutonium? Definitely.
But for a small body with no prior exposure, that is sufficient to cause an extended, intense high. Eating a piece of candy and then hours later feeling the onset of intense cannabis effects is disorienting, terrifying, and disturbing, especially when you have no idea why you're feeling it.
I fully understand what you're saying in that this child is not going to have life-long effects from this experience but we shouldn't discount the immediate effects or the shocking lack of responsibility by the parent.
And to be clear - I say "shocking lack of responsibility" because we're dealing with drugs, and in this case, I hold marijuana to the same standard I would hold any other drug. Cats can die from eating a single aspirin. I take prescription medicine that would probably be fatal to my child if she accidentally ingested one. I'm careful not to leave such medications where they can do harm, and it's right to expect that level of responsibility from everyone.