Unless you have a written employment contract with her which only allows you to terminate her for certain reasons and this is not one of them or requires some process or procedure to terminate her which you must follow, she is an employee at will and you could terminate her at any time, for any reason, including chronically bringing lice into a daycare center.Or you could suspend her until she clears up the lice and provides proof of it. Or putting the above together with an employer's right to put any terms or conditions the employer likes on employment, you can give her a written notice and/or a short agreement for her to sign stating that if she returns again with head lice, she will be terminated.