Sorry in advance for the long reply. I have written a few times about botulism since I nursed a bird through it for five days and there are very few survivor stories so I like talking about it.
They pretty much need to injest maggots or anaerobic bacteria to get botulism or deathly ill.
Let's assume they did eat maggots containing botulism. The only preventative measure that can help them after they ingested botulism bacteria is charcoal. They wouldn't start to get sick until 12-36 hours after ingestion. Then you would want to feed them charcoal and water mix since charcoal absorbs the toxin.
When I treated it the first time, I did give amoxicillan and charcoal, and my hen did survive it. However botulism bacteria, upon dying in the ducks digestive tract, release a toxin that causes full paralysis and thus death by asphyxiation. The toxin is released by the botulism bacteria dying, so antibiotics are said to worsen the effect because they kill the botulism bacteria, releasing the toxin more quickly. So no reason to give an antibiotic unless you determine they have a bacterial infection other than botulism. Pretty much nothing to worry about unless you see them fall ill, now you know you can give charcoal.
If you did want to be preventive, you could put some charcoal in their drinking water in the morning. By charcoal I mean fresh burned sticks or natural wood burned charcoal pulverized so the water has a greyish tint. If there were any toxin being released, that may mitigate it, however I don't think the effort is even necessary on your part.