Have any MLB bat boys gone on to become MLB players?

Ruben Amaro Jr. for one.

He performed batboy duties during 1980-1983. From there he went on to become a standout player at Stanford, from which the Angels drafted him in the 11th round in 1987. He came to the Phillies in a swap for Von Hayes in 1991.

http://jewishbaseballmuseum.com/spotlight-story/ruben-amaro/

Pete Rose, Jr. played in 11 games in 1997. He had 2 hits in 14 at bats.
Rose Jr. was often shown on national television during his childhood years as a batboy for his father's teams. He also appeared on a 1982 Fleer baseball card (#640) titled "Pete & Re-Pete; Pete Rose & son" with his father; he was twelve at the time. As a teenager, on September 11, 1985, he made an emotional on-field appearance live on ESPN to celebrate with his father after Rose Sr. broke Ty Cobb's record for most career hits.

https://en.wikipedia.org/wiki/Pete_Rose,_Jr.