I believe the active herbicide ingredient in weed n feed is a mixture 2,4 D and two other related herbicides, dicamba and mecoprop. They are some of the most toxic products it is still legal to buy and they have been banned in Canada, Sweden, Denmark, Norway, Kuwait, and probably other places. It is moderately toxic to humans, but mainly only in chronic exposure, such as agricultural workers. For you, being exposed to it one time, even if you managed to ingest some, it wouldn't be expected to cause noticeable symptoms. If it gets on your skin or in your eyes it can cause irritation. In longer term, chronic exposures it can be carcinogenic.
You want to be careful not to track it in to your house, if you have young children or pets.
I worry more about the environmental effects. It is toxic to honeybees and other beneficial insects. Consumption of plants treated with 2,4-D has killed horses and cattle. It is toxic to birds. If it leaches into streams, it is toxic to aquatic organisms.
2,4-D is not very persistent in the soil, with a half-life in the soil of 16 days or less. But note half-life means in 16 days half of it has broken down (sometimes into compounds that are more toxic than the original, which then have their own break down periods). In the next 16 days half of the remaining has broken down. So it takes a month for 75% to break down.
I did some reading and made a thread about toxic effects of common garden chemicals, here: https://www.helpfulgardener.com/forum/vi ... 11&t=57653
Re your plants: yes, the weed n feed is toxic to most dicot plants, which would be most of your flowers and vegetables.
Sorry, I don't have better news.
I don't think watering would help any, but if it is still just sitting on the surface, perhaps you can just brush some of it off?