How do I care for my young daughter on top of my spouse with cancer?

Good communication. Have a meeting with her if she is older that 7 years old. Tell her how you feel taking care of your spouse and the fears you are facing. Tell her that you would appreciate her to help in this or that. Less than 7 y/o...Get help with other relatives or other moms, or daughter's school. Get help from your religious or spiritual community. Get help for your husband too ( home health care or hospice).
Talk. I think the most important thing to do is talk. Don't assume she knows what is going on exactly or how she feels. In many locations there are special counselors that deal with families or kids and cancer. Look for local community resources to help -- if that help is wanted. Good luck.
We assume it's. You, rather than your spouse, but wheter you are the mom or dad, cancer is now part of the family, and a discussion about what it means gives them the opportunity to support you, as you suport them. Truth and open communication, good news and bad, varies with the age of the young one, but do not keep her in the dark. I hope you get well, and you grow stronger as a family.