I have full custody of my two children they have the fathers last name because he signed the birth certificate but the children don't use his last name they use mine how can I change it legally to my last name . I went to court and they told me I need him to sign to give me consent but I don't know where he lives what can I do ?

First its not a good idea to piggyback your question on someone else's. This can lead to confusion. You should start a new thread. So I've moved your question to its own thread.

You will have to make a good faith attempt to get his consent. This can include publishing an add the local newspaper or others methods. I would consult a local attorney to find out what qualifies as a good faith attempt in your area.

It starts costing a lot more money if you don't know where he is.
* and that also tells me you are not getting paid child support ?

But yes you will either need his consent or proof you make a complete search for him according to the standards of the court to find him ( and even then the court may or may not allow the change)