I am not sure that is true. It is Trump that has criminalized illegal entry of the USA and it is this decriminalization and associated charges that results in the seperation. But with an open mind, go ahead and please provide proof of what you say.
Congress criminalized it. He just ordered the laws enforced as written. Congress can show up and change the law at any time. Even to prevent a presidential veto.
But it makes a better photo op.
And the original picture that was taken and used by the media was proved to be one from 2014.
Now Trump could order ICE to stand down. Personally I think he should. But unless a mass rally shows up at the WH and threatens to burn it down I don't think he will.
He has decided to play chicken with Congress over immigration reform. And is using their own laws put in place after 9/11 to do it. If he chooses to sit back at some point the pressure will transfer to Congress. That is what he is banking on.