Hello dfbahr,
Thanks for your question, welcome to the community!
Sorry to hear about your deck woes 
If your deck is new as you say, you can forego the step of using a heavy cleaner for it. Heavy cleaners can take off layers of any stains or paint, so use a mild solution deck cleaner to make sure those trouble spots of checking and splits are free of dust and debris. I'm with you of using something now so as not to allow the cracks to further expand.
The Home Depot sells two things that would be the best solution to fill up the holes on and in your deck. One is an in-stock item and another is an online item that is better. If you have small cracks and checking, use the item we sell in our stores, the wood filler. For very large cracks and wood checking, use the online wood epoxy, both listed in detail below....
Depending on how large and how many splits you have on deck, you can use interior/exterior wood filler that we sell in our paint department that works great for small cracks in wood. Wood filler in itself is like wood putty, but it has cellulose wood fibers in the product, allowing for a stronger bond than a putty. Plus, you can stain or paint over wood filler.
Simply apply the areas affected with a small flexible putty knife so as to force the wood filler to fill the volume of the cracks and checks in the deck. Work on a dry day to allow the material to set properly, and you can stain and/or paint the surfaces when you are done.
Now...the 2nd option I was talking about earlier is an online only item, it is a wood epoxy paste. Use this item if you have significant splits and large areas missing. It is put on just like the wood filler above. It is one of the best wood filler products on the market. It's a binary (you have to mix the parts) formula that guarantees it will dry as hard as wood and be just as strong. Below is a picture and specifications of it...
To view the prices and customer reviews, the webpage where you can view it can be found here
Hope this helps you out,
aboveaveragejoe
