Thanks for reaching out, and welcome to the community!
To have your parent steps run in parallel, you can set the start trigger of the step to Run in parallel with the previous step. As long as each parent is running against different targets, each workflow will run in parallel while the child steps run sequentially.
As the warning mentions in my screenshot, if each parent step is running against the same target, the workflows will not run in parallel even with this set.
I hope this helps. Please feel free to let me know if you have any other questions.