UTMA gains

I'm the grandparent of two UTMA's for children ages 1 and 2. I sold stock in March and then reinvested back after the market tanked. I have received a 1099-B showing gains.  It appears that even with the money never leaving the brokerage firm, is still a gain. I'm shown as the custodian on the account.  I have a few questions : (1) If I fill for the grandchild, can I offset the gain with losses from my IRA account?  (2) If my son (father of the children) files for the children, how do I change the Payer Fed ID number so the IRS knows the 1099-B was transferred to him?