I’m pretty sure I’ve seen a girl in a dressy white girl’s pantsuit at First Communion in one parish church.
Some churches are more traditional and have a stricter dress code. It’s often for the sake of uniformity / so they don’t have to argue about whether little Hailey’s pantsuit looks too much like a boy or are leggings appropriate, etc.
The parishes I’m familiar with also allow a child to opt out of the First Communion ceremony and have their First Communion as a private family event at a different Mass if they wish. In such a case, there usually isn’t a strict dress code, although it’s the custom that the child and family will dress as befits a special occasion and not just appear in t-shirts and jeans. This might be an option if there is a big issue with one child not wanting to wear the prescribed outfit, although I would think that having to wear a dress for one day is not the end of the world.
I wasn’t keen on having to be fitted for a dress either at that age, but I did it just like I wore a dress for other special occasions my mom dictated that I dress up for, such as holiday visits with relatives who had gifted me a particular dress and Mom wanted them to see me in it. We have to do a lot of things as kids that we don’t want to do, such as go to school, go to bed at a certain time, wear a dress or suit occasionally, etc. It’s good practice for adulthood where we aren’t always allowed to wear our favorite clothes to work either.
I personally tend to agree that the uniform albs would be a great idea. Unfortunately, in the USA, somebody would find a reason to complain about that like they find reasons to complain about everything else. If there was no dress code, people would be complaining about some kids showing up in outfits they consider “disrespectul” to the Eucharist.