Bless you and bless that precious little baby!
If I was behind you (or anyone else) in line at a checkout with a baby, I would be smiling at the baby. I wouldn't be able to help it. ALL babies are precious! I wouldn't care if you (or anyone else) was using food stamps, vouchers or whatever, to pay for groceries, because it is NONE of my business.
I am someone that tends to worry too much about what other people think, so I totally understand where you're coming from. I know what you mean about wanting to say to strangers "it's not what you think."
My older daughter (mother of my grandson) works two days a week for her dad. She comes up to our house and works out of his home office. On the days that she works at our house, I take care of my grandson

yay

and sometimes I'll take him to meet my younger daughter for lunch, shopping, or just a stroll. She is a college sophomore, just turned 19, but looks even younger. I know for certain that when it's just the three of us, many people assume that Carter belongs to my younger daughter. I find myself going out of my way to let people know that she is his aunt and not his mom.
There is definitely a difference in the looks we get from strangers. When I'm out with my older daughter, people will engage Carter with smiles and grins. When I'm with Carter and my younger daughter, there are less smiles and more stares. I can understand people thinking that Carter belongs to my younger daughter. If I saw a mother, teenager and a baby out together, I'd probably assume the same thing. What I wouldn't do, however, is pass judgement on them. I would not frown at them or look for a ring on someone's finger. It is NOT my place to judge anyone else.
My daughter told me that it doesn't bother her because she doesn't really care what other people think. I need to start thinking the way she does! Ironically, my younger daughter and my grandson have the same big, blue eyes, and he looks more like her than his mom. My older daughter has brown eyes. Carter gets his blue eyes from his dad.
So, I definitely know what you mean about strangers passing judgement or making assumptions.