Friend putting me in awkward position

I have a friend who is constantly asking me to watch her son just because I’m only currently working part time. Her son is a very high needs toddler and cries the whole time she’s gone no matter how much I try to entertain him. I feel bad that I don’t want to watch him but it’s a lot for me to watch him and my daughter. How do I politely say no? Am I being unreasonable to not expect her to use me as a free babysitter? I also never ask to watch my daughter because I don’t think that’s her responsibility.