Question: I have $20,000 in a six-month money market certificate. During the next two weeks I have to make a decision on another rollover. With the yields so much lower than in the past two years, I am now considering a money fund--but wouldn't the broker's commission cost as much as the difference in interest? With the government now taking 10 percent of my interest I'm afraid my savings will dwindle.

Answer: You raise a couple of issues worth commenting on, even though my writing schedule won't permit me to get the information into print within your two-week deadline.

On the subject of money market funds, there are no broker commissions to be concerned about. Practically all money market funds are "no load"--no fee is charged either upon getting in or on getting out.

Regarding the 10 percent withholding on interest (to start July 1), you must realize that this is not a new or additional tax. When you file your tax return at the end of the year, you will take credit for every dollar withheld during the year.

If you don't have enough income to pay any tax, or if your tax liability for 1982 was less than the amounts given in this column a few weeks ago, you may request waiver of withholding on interest and dividends as simply an extension of the same pay-as-you-go system that has applied to wages for years. And if you had considerable income from interest and dividends not subject to withholdings, you have been required to make quarterly payments of estimated tax for years.

Frankly, I have very little patience with the scare tactics and emotional appeals used by the banking industry to drum up public pressure for repeal of interest and dividend withholding.

The only thing that will be lost by those who regularly report all their interest and dividend income will be the very small compounding that would otherwise be available on the 10 percent withheld--and that's 10 percent of the interest, not of the principal.

On the other hand, the government hopes to bring in hundreds of millions of dollars in tax revenue on interest and dividends that is not otherwise reported by tax cheats.

Of course, it will cost the payers of interest and dividends--but most of the numbers I have seen refer principally to one-time start-up costs only. Maintenance costs after initiation should be substantially lower than the amount of tax dollars that will be generated.

Employers, including the financial institutions themselves, have been doing the same thing for years--and more, with deductions not only for federal and state income tax and FICA (Social Security) but also for such things as U.S. bond purchases, United Way contributions, retirement plans and union dues.

I think the public would be better served if the savings institutions would gracefully accept the new rules and devote their energies to finding efficient ways to comply instead of spending money on statement inserts, mass mailings and media advertisements to stir up their depositors.

Their fussing and fuming has served only to persuade a lot of people like yourself that the government will now be taking another 10 percent bite out of your interest and dividend income instead of taking the same old nibble just a little bit earlier. TAX TIP: On several occasions I answered questions about the need for a dependent child to file his own tax return if he had enough unearned taxable income to equal or exceed the $1,000 filing trigger.

What do you do if the child isn't old enough to understand or fill out the return? Or perhaps even to sign it? In this situation, the IRS says the parent or guardian should complete the return for the child.

If the child is not able to sign the return, the adult should first sign the child's name, then sign the adult's name on the next line followed by the statement "parent or guardian for a minor child."

The parents may be able to claim a dependent exemption for a child who meets the support test even though the child also claims a personal exemption on the child's return. But the child's return in this case must be filed on Form 1040 rather than on one of the short forms.