While I don’t have a daughter, I have two caramel sons, and live in a similar setting in Europe. I’ve been in similar situations and have learned, that you should not second-guess yourself. You saw what you saw and felt what you felt. Valid and legit.
Do not let yourself get beaten up over these things, because I bet napkin-mom didn’t lose any sleep over it that night. The best way to avoid eternal self-analysis about what you could’ve said/done is to just say something.
I also find that the direct approach is always best. Coz as Judy wrote above, napkin mom just doesn’t get it that what she did was not ok, and lines up perfectly in the category ‘racist’. I recommend staying polite and friendly.

This is how I would have played it:

Me (pointing at the napkins): So what’s wrong with those napkins there?

Napkin-mom: Nothing, I… (as she realizes who you are and what she just did)

Me (cutting in): Right. I see. Didn’t expect that from you. Well, let’s see if we can stash these in the take-home/ goody-bags for later.

Then the ball is in her court, to defend her actions.

If the kids didn’t notice, then it isn’t your job to teach them racial diversity. But napkin-mom sure could use some training.