Are you sure they actually ate it? I would think there is a better chance that they just brushed up against the plants and dispersed it that way. I've never used sluggo but I'm assuming you just dust the plants with it? Are they showing any symptoms?
I found this site that talks about how toxic Iron Phosphate sluggo can be;
http://www.hostalibrary.org/firstlook/RRIronPhosphate.htm
Although I'm not posting it because I agree with it, I'm just posting it because he discusses how the poison works. Apparently it contains EDTA which is a chelating agent (it binds to metal) and will bind with the iron in the iron phosphate. Supposedly this makes it toxic to the slugs. The reason I don't necessarily agree with him that it is so toxic to all animals is because EDTA is actually the treatment for metal toxicity (I know this because I had a bird who was very sick and was prescribed EDTA for suspected metal toxicity).
Heavy metal toxicity is a serious concern with birds, they can perish rather easily from it. BUT it usually occurs from them eating something metal that they cannot pass naturally or from a long term exposure to something that accumulates in their bodies. Something like a one-time dose of powder I would imagine would pass fairly easily. They may get some effects, but not
enough to kill them.
I could be completely wrong, but based off this, I wouldn't think they are in dire trouble. I am not a vet though and I'm just coming to this conclusion from a quick google search (and a little bit of past experience with metal toxicity). If these ducks are dear to you, I would take them to the vet regardless. Something like this, if it is toxic to them, will need to be treated quickly.
If you aren't going to take them to the vet, I would observe them for symptoms (lethargy, not eating, changes in poo) and treat accordingly. Not sure how I would treat them at home for this kind of poisoning though...