If i lets someone file my kids and now i want to claim them to get stimulus benefit how can i fix that

You should be filings accurate tax returns.  If you allow someone to claim your children as dependents who does not meet the qualifications, then both you and the other taxpayer will be subject to back taxes, penalties, and you can be barred from EIC for a minimum of 2 years if a false EIC claim was involved.

For 2019, if you did not file your return yet, you can file a correct tax return.  If it conflicts with someone else, the IRS will investigate both of you.  

The stimulus payment is technically an advance payment of a new credit that will be on the 2020 tax return.  If you file a correct 2020 tax return and qualify for a larger payment than you receive this Spring, the extra will be added to your refund next tax season.  The IRS still may investigate if the same children qualify for a stimulus payment on one person's tax return in 2018 or 2019 and a different person's return in 2020.