So, I know it's OK to share your account with one child who is under 18, but does it matter what toon he plays? Can he play one of mine?

Here is why I ask; you can't share with your spouse but you can with a child. If a GM will ban you for sharing with your spouse, how do they distinguish that from sharing with a child? What's the deciding factor? Best judgment or do they look closer at who is playing what toon (i.e. ToonA, ToonB are being mutliboxed on Friday but appear to be playing by different people on Saturday)

My son is only 7 so I only let him play an hour or two a week but I think it would be fun to let him play one of my toons during an instance run. I just want to make sure that's ok, and/or it is not going to look suspicious.