The Eucharist has a nuptial dimension in Christ, in much the same way as Baptism is about death and rebirth into Christ. The Eucharist now is a foretaste of the Wedding Feast of the Lamb, at the end of time.
So it is custom in Europe and elsewhere that girls dress up like Christ’s Bride, the Church. I do not know when that started, but French materials showed white bride dresses and veils on First Communion girls almost as soon as brides started wearing white, in the 19th century. What people did before, I do not know.
(If you want your little girl to grow up dreaming of getting married in a pantsuit… Well, geez, even then there is such a thing as lace and veils. But I was a tomboy, and even I wanted the white dress for FHC.)
Boys dress up as Jesus the Bridegroom. It is just not as obvious; because a groom can wear a dark suit, but so do other men in formal situations. Filipino-American boys often wear those formal shirts that are like suits in the old country.
Wearing albs is acceptable, as it is the vestment of any baptized Christian; but it wimps out on the nuptial dimension. Any formal clothing is also acceptable, but… Why ruin the symbolism, and yet want to have your kid make FHC in the same Mass?. Informal clothing is rude, but does not invalidate anything… But again, why? Even inexpensive clothing can show care.
If you do not want the symbolism, any day can be your child’s FHC – just ask the pastor and do the records paperwork.
So yes, there is a theological and religious reason.