In Logan, we know that the kids in the facility grew up in there along with Laura.
So why do some of them have different accents than others? If they grew up in the same place, shouldn't they have the same accent?
All the kids weren't from the same place, or at least not the same unit of the building. Rictor is obviously older and was not part of the kids that Gabriela had recorded. There were more kids than that. And keep in mind these kids were not given normal education. They were regarded as weapons, experiments and denied basic human contact. So they picked up the language ad-hoc. And there were mixes of accents around. The nurses may have been Mexican, but at least Gabriela spoke English. And all the guards, the Reapers, had a mix of American Accents, heavy ones at that. Dr. Rice did as well, a more English accent. Due to all of this, they couldn't be expected to have a single accent. Remember, Laura English is very bad, much worse than Gabriela. It wasn't formal learning.
Note, Gabriela is a Mexican Actress, and Laura is a spaniard. Their normal Spanish do not sound the same anyway, and you can hear that in the film. Laura is a 10 year old so obviously we can't expect her to neutralize her normal speaking accent yet.
It could just be a plot hole, but one plausible explanation could be that different accents are due to each child's primary caregiver at the Transigen facility. One scene showed that at least some of the children were kept in relative isolation, so any accents they had would be learned from the doctors and nurses taking care of them. In Laura's (X-23's) case, that would be Gabriela, the nurse who helped her escape from the facility.