Cor, can I have one of them assistants then hard? 
I tell my students to proofread for each other as an exercise. It works well. Yes, there are marks for it, but there are also dictionaries made available. The idea is not to be perfect without trying, it's to learn how to spot mistakes - with the awareness no-one will do it for you in the exam.
I think, btw, that the idea anyone can spot where an argument needs developing is pretty patronizing to your adult children, unless you are a specialist in the same area, in which case it's simply unfair and detrimental to their learning process.
A student I had last year had a boyfriend doing the MA (ie. two years ahead of her in his studies). She was very open about it. However, it was extremely obvious when he'd misguidedly told her where the mistakes in her argument lay. She'd come up with an essay that was a patchwork of inappropriately MA-style argument, bits elaborated where I knew she did not have the skill to do so, and a poor understanding of how the whole thing might possibly fit together. She was better writing on her own and just discussing the topic with him, I suspect. Could be wrong though as you never quite know how someone works in private.