I think 42 teeth is the maximum cassette size for a 20” bike. I built my son a 20” bike with 2.125” tyres and an 11-42T cassette driven by a Box 4 derailleur and 32 tooth chainring, and it works perfectly and is great for climbing and difficult terrain. Tech support at Box told me with any of their wider cassettes the derailleur would strike the ground/tyre on a 20” wheel bike.
Now I am looking to upgrade my son to a 24” bike. I have seen 24” bikes for sale running 9 and 10 speed 11-48, but does anyone know if an 11 speed 11-51 or 12 speed 10-50 will work please?