Niacin- Do you know the niacin content of their feed? All ducks need niacin, but how much depends on age. If your feed has under the recommended amount for ducklings I would supplement with extra niacin until full grown. If it has the correct amount you wouldn't need to supplement unless you saw signs of a deficiency.
Keeping the brooder dry- I buy the cooling racks that come with the baking sheet and set their water on it for ducklings. You want the kind with the mesh cross hatch that's at least 1/2" square or smaller. Do you have a picture of their waterer? I have found that an old milk or water jug with holes cut just big enough for them to get their heads in and out work great as waters. The water stays cleaner too and it's harder for them to tip them.
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Being moved outside- That depends on your daytime/evening temperatures and if you have the ability to run electric (even just an extension cord) to supplement heat if needed. Generally ducklings need to be kept at 90-95 degrees (I start them out at around 90) for the first week and then you bring it down around 5 degrees every week from there. So take their age in weeks, multiply by 5 and then subtract that from 90. It doesn't have to be exact temperatures, but it gives you a good estimate. If your outside temperatures maintain around the number you get you don't need to supplement with heat, but if outside is colder then you will need to have a heat source.
Coops- Check craigslist for coops. Anything pre-fab is garbage. On my local craigslist there are coops all of the time for cheap. Even people selling whole sheds in your price range.
as for craigslist… it was mentioned to me by another member also and I am definitely going to check it out. Thanks so much for all the advice.
We still enjoy watching them grow and explore.
Thank you for the advice! We were told not to really give them lettuce but I might try broccoli heads chopped up. As for feeding schedules… mine are still young (4 weeks old) and I was also told to let them eat and drink as much as they want all the time, but at what age should we start to wean them onto a meal time only schedule? Hope I’m not too late to start too. They are still indoors so it doesn’t bother me either way, but once they are able to go out in a coop, I don’t want any trouble getting them in and out of it. Oh! Before I forget to ask again, what age can they go outside for the very first time? Isabella begs me to let her take them into the back yard for just a few mins because she read that they eat grass and bugs etc… she says her babies need their mother nature foods LOL 
(she makes us laugh). She is also determined to sit in the tub with her swimsuit on while they have their bath time. I said I’d have to look that up because I’m not sure it’s safe for her when they often poop in the water. 
Her solution is that she will take a shower as soon as they are done. 
My goodness we have sooo much to learn.
