Find the chicken-loving veterinarians at your nearest school of veterinary medicine. Surgery on chickens is tricky because of their fast metabolism of drugs which requires expert anesthesiology. I have found chicken loving vets at the nearby vet school. The prices of the vet school are often more reasonable too.
The vet school will have a capacity to do some things which are beyond the (safe) capacity of ordinary vets, who will sometimes tell you they can do it,but in reality they are not qualified. I learned this the hard way,having a leg amputated from a chicken in a hurry when I learned --too late-- that chickens can live fine on one leg. After the chicken died on the operating table at my local vet, when I talked to the vet school professor who is the chicken lover, he commented about my vets, "So they thought that they were capable of doing that?" He had been my vet's, vet school professor, and his message was clearly that they were over-reaching to even think that they should attempt to amputate a chicken's leg, which was clear from the outcome. Unfortunately, I had nixed a possibility to do it at the vet school previously, thinking that the chicken would not be o.k.with one leg, and had instead continued to try to cure the bad leg, until deciding that the chicken was going to die, so trying to have my local vet amputate in a hurry.
From what I know crowing is a feature of testosterone, and rarely hens can convert to crowing and even grow plumage like roosters and then convert back to hens. So I would guess that canonizing a rooster would greatly reduce,if not block its crowing.In humans, there is only one other gland capable of producing a lot of testosterone, the adrenal glands. There are some other tissues in humans that can produce small amounts of testosterone. My --guess-- is that this is similar in chickens, that a rooster who has broken adrenals, or exhausted adrenals, might over produce testosterone due to hyper-stimulation of the broken adrenals even if canonized. My aggressive roosters have been the ones I have suspected of having weaker adrenal function,but my race is Brahmas who are ordinarily pretty mellow.
Good luck, I am sympathetic to your daughter's plight, as I am sure she loves her "roos". The person who commented that the caponizing should make them safer and less aggressive as a child's pets, had a good point. So did the person who commented that it would give them a chance for a good life. Teach her not to expose her face near the roosters as they mature. This is just common sense. A rooster can even injure an adult who gets his face into a vulnerable position.