I was a childminder for 15 years, I never had an assistant but lot of my friends did. Working with another person like that you really have to gel with them as you are spending a lot of time together in what can be a stressful/busy situation . Duties include - setting up activities, playing, cleaning, personal care, schools runs, outings, writing up observations, communicating with parents, cooking.
You would need to check if you would be self employed or employed, what your contracted hours would be. What happens when the childminder is ill or on annual leave. How much annual leave you would get and when you can take it.
Hopefully someone who has done the role will come along to give more specific advise. I loved being a childminder and working with children. The hardest bit was the parents, chasing payments , unreliable time keeping. Also the paperwork, there has to be lots of evidence to show what your doing so observations, planning activities for development, linking to eyfs. As an assistant though you would probably be less affected by those things. it is also lots of fun and very rewarding.