Hanes Baby Long Sleeve T-shirt, Ultimate Flexy Knit Tee for Boys & Girls, 4-Pack

Q: For a 19 month old who is 23 lbs, 32 inches height (50th percentile), will the 18-24 month size be too small?

My son was about that size back in Oct when I bought the 12-18 and they fit him fine (they’re kind of shorter than expected so maybe you should go with the 18-24)

Q: Does this clothing contain BPA?

I don't know about BPA, but they definitely feel plasticy and are especially awful when they're damp. Do not recommend. H&M sells similar shirts made of cotton and they're much better--although they run very large FYI

Q: Do these work as undershirts or are they too bulky? Looking for something to go under the tons of short sleeves we have this winter.

Yes, these would be fine under a short-sleeve shirt. I've actually had my kids wear these in the summer to protect their skin from the heat, so they are a fine under-layer but would not recommend as a primary warm shirt.