My ex husband and I divorced 4 hrs ago and I got remarried 2 hrs ago. I have full physical and legal custody
Of our 3 kids, but I want to change their last name to my husbands. Can I do that and, if so, how do I go about changing it?

Yes you can. I did the same for my son. I went to the County Building, Or where ever YOU would go to buy the child's Birth Certificate, and let them know you want the last name changed. It will cost though.