I find the argument that he’s hesitating to pardon himself and his family because it might upset Senate Republicans to be laughable. He could pardon every inmate in Gitmo and there wouldn’t be 17 Rs to convict.
I thought I read somewhere on the boards here that if he pardons his adult children, then they can no longer be protected by self-incrimination laws or somesuch, in the event they be pressed to testify against him, or each other. I do not remember the specifics, but perhaps someone with more legal experience can explain it better.
It’s the Fifth Amendment to the Constitution, which defines the right not to self-incriminate.
If one is pardoned, one is no longer in legal jeopardy, and one can be compelled to testify.
This is clearly not in Trump’s interest.
‘Compelled to testify.’ I’ve always been a bit slow on the uptake, but this compelling consists of something like ‘Talk, or we’re throwing you in prison/fining you lots of money?’ What’s obvious to some or even most sometimes escapes me, and thanks for not patting me on the head. 
If the court orders you to testify because none of the exception reasons (you are the accused, you’re the spouse of the accused, you’re a member of the clergy who received confession in confidence, etc.) applies to you, and you refuse, you can be found in contempt. So, yes, punishments for contempt range from monetary fines to imprisonment, or both, depending on circumstances and judicial discretion.