I do about what Tom does, file a return for the child. I too have seen IRS letters based on Proceeds, even though the major brokerage reported Basis to the IRS. It's so much faster and easier to file the return for a child than to respond to an IRS letter.
Filing for children (and the elderly) also can prevent ID theft, or in one case with a client, uncover ID theft early.
Even though if it's only interest/dividends and can go on the parents' return, it has saved my clients a bit when reported on the child's return.
Years ago in my HRB days, we got an IRS letter looking for a child's return when the interest had been reported on the parents' return where the child was a dependent. Time-consuming to deal with. And, unhappy clients getting an IRS letter!
I charge a flat amount for children, sometimes separately and sometimes on the parents' invoice.