I'd see her words as clumsy maybe, but honestly, not rude. Considering what she saw, I'd think that she was just trying to help out a mother.
How lovely of her!
Sometimes I get caught for words and don't necessarily say the exact right thing, and I'm sure most of us are the same. I've debated with myself on occasions about whether to help out another mum in a grocery line, and rehearsed over and over what I will say, deciding that everything I come up with is clumsy.
No matter how you try, in that sort of situation you can easily embarrass the person that you are trying to help.
I'd try to see it for what it was - a kind, generous gesture. Gosh, $40 is a lot of money for someone to just hand over to a stranger! If she were a snarky person, she'd have just made a snarky comment, surely? Not handed you a significant amount of money!
I don't think he looks skinny at all. He's super-cute! Seeing those pics, I don't imagine that she was thinking he was skinny, just that you were struggling to buy groceries. If I see someone who seems to be struggling to buy groceries, I try to help out. It's not snarky, it's just that I can afford groceries, and don't believe that anyone should be in a position where they can't afford the basics in life.
Maybe the soda swayed her opinion of you, but maybe it did not. Either way, she was a very generous person, and I think it would make you feel more comfortable to just think about her generosity, not her words.
(And btw, although I don't post in special needs, I've had my share of judgements over my kids, so I do understand that. I also know that on occasions I've jumped to the conclusion that someone is judging me harshly, then they have come out with a really kind comment or gesture that shows that the opposite is true.