When can I change my children's last name with out the fathers consent.

Until they are 18 you generally can't. To legally change the name you have to obtain a court order and they will require his agreement.

I changed my sons name in Ohio when he was 2 without his father knowing. I called the court house and they posted a name change request in the local court newspaper. (A newspaper that typical people don't read) and if no one contests it within 30 days they give you a court date and on that date it was changed. My sons father was no where around at the time, but just wanted you to know that there are ways!!

I changed my sons name in ohio when he was 2 without his father knowing.

And if the father surfaced and took you to court, he would probably be able to get the name changed back. The law requires you to make a good faith effort to contact the other parent to get their permission. Different jurisdictions define good faith effort differently. A common one is posting a legal notice in a local newspaper.

Your scenario may not be similar to the OP's. There was no mention of whether she is in contact with the father or not.