Someone filed my child illegally. How can I find out who did this and how can they be stopped?

You merely claim your own child, and provide the IRS with the information they need. They will deal with who is doing it illegally.

1. it has to be someone that would know the chid, and have access to the child's social security number. Normally it is a close family memember

Redundant post, but Fr Chuck's answer is the correct one.

Someone filed my child illegally. How can i find out who did this and how can they be stopped?

How did you find out that someone is claiming your child?
Normally, don't worry about this. File your return and claim your child (if you meet the requirements). If someone else has also claimed your child, you will get a letter from the IRS. Then the "someone" must amend his/her tax return and pay interest and penalty of the amount due.

Agreed. The IRS will send you a letter and may hold up your refund while it gets resolved, but if you are the custodiual parent who provides more than half of the child's support, YOU will get the exemption.

I'm guessing that it was a efile that showed up someone had already claimed the child...

I've heard of that causing the return not to be accepted in that manner for the quick refund. It might then have to be filed via a different means.

It may have also been a simple keying error with the child's SSN.