Case law stipulates that the Custodian, acting as the Fiduciary on the child's behalf, must make all decisions for the benefit of the child. If the custodian is not providing you with the 1099 for filing purposes, you should ask if they are filing the tax return for the child, and if so, what are they going to withdraw from the UTMA for the charge to file. There should be a clause in the original UTMA document stating how the reporting is to be done and who is entitled to view what is in the account.
I would ask the Custodian why they are not providing you with the 1099 in question and if you believe their reasoning is flawed perhaps consult an attorney.
@BBB10