Retirement tax questions

It was truly a grantor trust, which became irrevocable when my mother died.   So there will be a partial year 1041 for the trust.   I'm just not sure how to handle that taxable income that came into the trust while it was still a grantor trust.   The payer (dad's former employer) would not pay out the benefit unless the trust had its own tax ID number.   So I got one, a couple of months before mom died, knowing I would need it anyway.   The 1099 they will send at the end of the year will have the trust's tax ID number.  But I believe it should really be passed through to their personal tax return.   I'm just not sure how to do that.