@Pauna Soo We do not know what help you are asking for. No one here can get your children's Social Security numbers for you. If you do not have them, you need to contact Social Security yourself and provide the information that they will require in order to give you the children's Social Security numbers.
If your problem is that the other parent has claimed the children, and YOU are the custodial parent who should claim them:
Someone else claimed my dependent and my return was rejected
If someone else claimed your dependent already, then your e-filed return will be rejected. Print, sign and mail your own return which claims your dependent. Mailed returns are processed differently by the IRS. It will take some time (maybe even up to a year) but eventually the IRS will contact both parties and sort out who could rightfully claim the dependent. The person who wrongfully claimed the dependent will face repayment of refund $, plus penalties.