Washington(CNN) Some Social Security recipients must tell the Internal Revenue Service how many dependent children they have before Wednesday at noon in order to get their full stimulus payouts.

If those people don't take action, they'll still automatically receive $1,200 -- but will have to wait until next year to receive the additional $500 due per dependent, according to the IRS.

The agency issued the guidance Monday afternoon, giving less than 48 hours' notice. It's a small window because the IRS is sending stimulus payments to this group soon.

But the tight deadline could impact the families of an estimated 600,000 child dependents, according to an estimate from the Center on Budget and Policy Priorities.

"Many of those people could end up missing out on the $500 per child stimulus payment altogether, given the difficulty of either meeting an extraordinarily tight deadline or having to file a full -- not simple -- tax return for 2020 to get the payment in 2021," said Samantha Jacoby, a seniors tax analyst at the Center on Budget and Policy Priorities.

Many Social Security beneficiaries are not required to file tax returns because their income is below the filing threshold. But if they miss Wednesday's deadline, they'll have to file a full return next year to get the money.

Social Security beneficiaries who have already filed tax returns do not have to take any action in order to receive money for their dependents because the IRS already has that information on file.

Those who aren't required to file full returns must use the IRS's online "non-filer" tool to input the information on dependents. Children under the age of 17 qualify for the additional money.

The April 22 deadline applies to those who receive Social Security retirement benefits, survivor or disability benefits, or Railroad Retirement benefits.

Some other federal benefit recipients who don't normally file will also have to use the online tool to input information on dependents. Those receiving Supplemental Security Income or low-income veterans who receive pensions form Veterans Affairs will have more time to input information on their dependents, using the same tool.

That's because their stimulus payments are going out a little later. The IRS said payments will automatically be sent to Supplemental Security Income recipients in early May. The payment schedule for those who receive veterans' benefits has yet to be determined.

Earlier guidance from the IRS suggested that those who receive federal benefits and don't normally file tax returns would have to do so. But the agency has revised that guidance and is working with other federal agencies to get the bank account or address for those people so that it can automatically send them the stimulus payments.

They will receive the stimulus payment however they normally receive their federal benefits -- either through a direct deposit in their bank account or by a check in the mail.