Historically, labor unions opposed child labor for reasons that a) they should be in school learning things and b) they work really cheaply since they're unskilled. Unions in the 1800s were pretty open about why they opposed child labor, and they always mentioned the corrosive effect of an underclass of unskilled labor. So yeah... people might not think about it much now. But if you repealed child labor laws, I'm pretty sure the unions would be trying to fund high schools EXPLICITLY so they could shape the structure of labor participation by cohort.