For what you want , Distributed.jl should have you covered I think. addprocs supports the keyword exeflags which allows you to specify the amount of threads the worker process should be started with.
Is there a reason you require separate processes for though? Should these workers be distributed to different machines or do you need to use external libraries that can’t be parallelized otherwise? If not, then I’d recommend just using a single Julia process with however many threads you can afford and letting Julia’s scheduler handle the rest 