Set a callback for when a certain process is spawned

The script I'm writing launches process A, which after an uncertain amount of time, launches a child process B. I need the script to execute a command C which depends on process B. But B might not be there at call time! **How to wait until a process with a certain given name is spawned?** I reckon I could just blindly poll/wait inside an infinite loop but maybe there's a fancier solution...