I wouldn't get too upset about it, on a personal level, at least.
Yes, it's maybe been presented a little bit literally in cash just being shoved in a card, but you don't know what's going on in her life, like you say, she mentioned her Dad is unwell, but you don't know how unwell he is, perhaps it was too overwhelming to organise anything else, or she was stumped, or just couldn't be bothered, some people are good at offering to do things, but not good at asking for help - so I'm sure she really appreciated your message asking her if you can help with anything.
I can totally relate to your frustration though, it's not been executed as you had hoped, or would have done yourself, and she has unwittingly left parents out of the collection, the thing with collections is, whichever way they're done, you won't please all of the people all of the time.
I'd do as some others have suggested above, a nice wee thank you card, a little homemade card or picture from DS, and the Teacher will be delighted.
Try not to get too upset over it.