The child doesnt have a swimsuit, all outfits are made to be worn by all characters, and scale to whatever size the character is.
But, yeah, some of these go way beyond OG Xenoblades original swimsuits, or even Sharlas more impractical heavy armors, which were actually swimsuits, and more into eyebrow raising territory. And since any character can equip pretty much anything (I think I might die of laughter asphyxiation if I see some streamer with a huge male muscle chunk running around in a string bikini) well, lyn can wear it too.
Its to the point I seriously doubt those 'armors' it will make it out of localization without change, and thats not even considering Lyn as a reason.