Dependents can only be entered on one tax return. If your mother provides over one-half of the support to the children then she can claim the children on her tax return. You have to amend your tax return removing the children from your tax return.
If she also provides over one-half of your support and you do not have gross income in 2020 of $4,300 or more then she can probably also claim you as a dependent.
See this TurboTax support FAQ on amending a tax return originally completed and filed using the 2020 TurboTax online editions - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/01/27577
The IRS Form 1040-X for amending a 2020 tax return is scheduled to be available on 03/25/2021 (subject to change)
