SWiFT for Metastorm BPM can do this with the standard skins we use. However, child-forms currently appear as a pop-up. It is possible to have these embedded and we are currently working on this 'one window' functionality. It is on our road map for a later release.
I have also attached some examples below to give you a better idea:
If you do have any further specific requirements including building a custom front end, please do send me a message. I'm more than happy to discuss with you.