Trump should make sure that the sanctions does not result in a decline in the Iranian crude production. Else it will affect the Americans more than anyone, since they are among the top importers of crude oil.
China doesn't seem to care about any American sanctions imposed on Iran anyway. They had been buying Iranian oil when the latter was sanctioned up to the hilt. I don't think they are going to change their stance any time soon, especially if we take into account the stuff that (Trump is smoking) Trump is spouting out (in what concerns China). On the other hand, Russia was rumored to have been secretly buying Iranian oil previously and then reselling it as their own to European countries. Why couldn't they do it all over again?
After all, the US has no access to the Caspian Sea to check that trade (unlike the Persian Gulf)