Don't worry. Here are a few tips to cheer you up and help you face the rough weather during the first few weeks:
Don't hit the panic button: If the job is not to your liking and it isn't what you expected, you may instinctively hit the panic button and quit the job or look for new job. Don't do that. Don't make any hasty decision. Find out why you feel that you have been duped. Analyze the situation. Make a list of what you were told and what you are now asked to do.
Talk to your supervisor: Your supervisor or manager may not like to be understaffed. Talent retention is an issue nowadays. It is wise to speak frankly with your supervisor. Tell her that you have not expected to do what you are being asked to do. Ask her to tell you what your duties and responsibilities are and try to persuade her to check on the agreement you have with the company. If she is unable or unwilling to sort out things, seek help of the HR people.
HR help: There should be a solution to your problem. None other than the HR people understand how difficult it is to find capable and sincere employees. There may be many reasons for your dissatisfaction. Express your feelings to your manager or to HR, you may find a satisfactory solution.
Aren't you able to take care of your kid because of long hours? You may seek flextime. If you think you should get additional training, ask them how to get it. Is your job less challenging? You can ask for challenging responsibilities.
Even if all efforts have failed to satisfy you, still you can look for future at the company. A good employer always wants to keep talented an employee who can be used in a different capacity.
Only option: When you are not satisfied with the job, better you look for another opportunity. Find a new job immediately so that there won't be any gap in your resume. Even in the worst situation, treat your employer with respect and professional courtesy though you might not have been treated that way. Who knows you may have to rely on this company.
To avoid a situation like this one in future, better ask for a comprehensive job description before you accept the offer. A written understanding will help you and your supervisor know what your duties will be. Don't hesitate to ask your future manager about the working hours so that you will know what will be your professional life.