It's not as black and white as you describe. The Chinese government helped impose UN sanctions on North Korea but there were still Chinese firms that ignored them. Obama imposed sanctions on the Chinese firms that were found to do so. Much of the the trading between China and North Korea is illicit and I suspect that it will continue. 

And I doubt anyone can "force" China to do anything it doesn't want to do. But let's assume that Trump can. Then Trump would be forcing China to do what exactly? Have North Korea give up its nuclear weapons? Never happen! North Korea will use its nuclear weapons before it abandons them. And China knows this which is why it too treads lightly when it comes to North Korea. 

Sorry but I think Trump is again displaying his gross incompetency for the office he holds and in this matter, it's dangerous for humanity.