Up to 350,000 families nationwide have not received an additional rebate amount for their children even though they’re eligible, the Internal Revenue Service said yesterday.

As a result, the IRS plans to mail special rebate checks to those families in July to make up for the additional amounts they should have received but didn’t, IRS spokesman Terry Lemons said.

Taxpayers affected by the snag need not take any additional steps; the IRS will correct the problem internally and issue the checks automatically, Lemons said.

Families affected by the problem did not receive the additional rebate amount — of up to $300 per child — for one of two reasons: Either they didn’t fill out their tax returns completely, or they — or their preparers — used tax-preparation software that didn’t capture the necessary information correctly, Lemons said.

The IRS became aware of the problem after fielding inquiries from taxpayers and tax preparers, Lemons said.

I learned of the problem when a woman from Cranston e-mailed MoneyLine:

Q: We have one child, and should be getting $1,500 in a stimulus rebate. When I checked the IRS Web site, I found that we are only getting $1,200, and they show zero qualifying children. How can this be? The tax return is correct, stating three [exemptions]. My 2007 taxes are paid in full. Why is this happening? …

— C.C., Cranston

A: I visited the IRS’ regional headquarters in Boston on Wednesday to try to get answers to some rebate-related issues that MoneyLine readers have raised.

When I asked IRS spokeswoman Peggy Riley about your particular issue, she told me that a number of people have raised similar issues, and that the IRS was aware of the problem and planned to take steps to fix it.

Yesterday, the IRS’ Washington office called me to confirm the problem and outlined the steps the agency will take to resolve it.

Some background:

The IRS calculates rebates based on the information provided on your 2007 federal income-tax return, the one you filed this year.

If you chose the “married filing jointly” filing status on that return, you’ll generally receive a standard rebate amount of between $600 and $1,200.

If you chose another filing status on that return, such as single or head-of-household, your rebate will range from $300 to $600.

However, you’re generally eligible for an additional rebate — of up to $300 — for each child you have who was under 17 as of Dec. 31, 2007.

And there’s the rub.

The IRS projects that about 36 million returns are eligible for an additional rebate amount, and nearly all will receive the correct amount.

But up to 350,000 of those returns either haven’t received the additional amount per child, or will not receive it in the near future, because of the following problems:

•Incomplete Returns: When you list your child as a dependent toward the top of your U.S. Form 1040 or Form 1040A, you’re supposed to include the child’s name and Social Security number, and list the child’s relationship to you. You’re also supposed to check a box indicating whether the child is a “qualifying child” for purposes of the federal child tax credit.

In general, if you have a child who meets the rules for the federal child tax credit, you should also receive an additional rebate amount for that child. But some people forgot to check that box. As a result, the IRS did not process an additional rebate amount for that child, Lemons said.

•Tax Software: If you — or your preparer — used a certain type of tax-preparation software, the software may not have “captured” all the information necessary to process the additional rebate amount for children.

As a result, the IRS did not process the additional rebate amounts for the children listed on those returns, Lemons said. (The software was produced by CCH Inc. and by Petz Enterprises. The vendors have been working with the IRS to resolve the problem, Lemons said.)

Deborah J. Hadden, former president of the Rhode Island Association of Enrolled Agents, which represents federally licensed tax practitioners, said she wasn’t surprised that a snag would develop, given the scope of the rebate program.

Overall, the government is processing about $107 billion in rebates this year to be issued to about 130 million households as a result of a new federal law intended to help stimulate the nation’s slumping economy.

It’s almost inevitable, then, that a problem would arise, said Hadden, owner of BRL Associates LLC, a tax-preparation firm based in Westerly.

What to do? First of all, remember that the IRS has been able to correctly calculate rebates for nearly all the millions of people who are due one, Lemons said. (And if you haven’t received your rebate yet, be patient — the process is still going on, and will continue through the end of this year.)

If you’re among the estimated 350,000 taxpayers who are involved in the snag described above, you needn’t take any steps, Lemons said.

The IRS has identified the problem and is taking steps to resolve it, he said. “It will all be done automatically,” he said. “We don’t want people to worry about it,” he said.

The IRS plans to mail checks in July to those who are affected by the problem, he said. (The agency can’t use direct deposit for these payments, he said.)

Lemons declined to estimate how much money will be distributed through this special round of payments. However, assuming that 350,000 families are affected, and each has at least one qualifying child, the amount could exceed $105 million.

So here’s the bottom line: Most people have already received or will soon receive the full rebate amount to which they’re entitled.

But some families have been caught up in a snag. If you’re in this situation, don’t worry; you should receive, in July, a check from the Treasury to make up for the difference between what you have received and what you should have received.

“They’re still going to get it in July,” Hadden said. “It’s going to work out well.”

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