There are some situations, and I think this is one, where the answer is 'make a decision, ANY decision, and then go with it and DON'T CHANGE YOUR MIND!' I would have done what you did (but then, I know that both the Chicks, once they start to boak, will keep going until the tank's empty) but I suppose I can see that there might be an argument the other way. If your instinct is to grab and run, then grab and run without second guessing yourself. Don't stop to ask 'are you going to be sick again?' because the answer 'no' is probably more hopeful than accurate.
Oh, and if you were worrying about what the people around you might have been thinking, don't. The ones who have had children were either thinking 'oh, poor girl' (meaning either you or the child) or possibly 'there but for the grace of Deity go I' with a side order of 'remember when Joshua did that in church? Yes, I do, thanks, and I wish I didn't'. The ones without children were probably thinking much the same, adapted to 'your friend Susie's Joshua'.
I wouldn't worry too much about what FMIL thinks; I would be more worried about what your stepdaughter thinks - 6 is old enough to be absolutely mortified at that having happened in public. But I'm sure you have that in hand!