Yes, your employer may force you to take a different shift, even if you do not have childcare available for that new shift--and even if one of your children has a medical condition. The law in the United States does not force employers to accomodate the needs of employees' families, no matter how deserving--the employer is not required to consider your childcare needs at all. They can change you to a different shift, and if you can't work that shift, terminate you.