Ironwood, could you clarify something because I'm a bit confused about your stated goals. On the one hand, you want a shelter against blast forces, although you only expect downwind effects, meaning fallout, I assume? I don't mean to bust your chops just for the sake of it, but just trying to get a clearer focus of what kind of situation you're trying to prep for. And I'm certainly no armchair expert on radiation and nuclear war scenarios, so for what it's worth--

Unless some nation-state is attacking us with ICBM's from a long distance, I wonder if it is practical to plan on a scenario where there is enough warning seek shelter from blast effects? Seems that a relatively small, low yield nuke device snuck into a high value target area and exploded at or near ground level surreptitiously would be a more likely scenario today. Sounds like you don't live in a high value area, like DC or Manhattan.

Have you considered making evacuation your primary response? If downwind effects are your main worry, I would personally get out of Dodge, especially with young kids. Unlike a full scale nuclear exchange that we all feared during the Cold War, an isolated nuke is going to leave plenty of areas to escape to. There isn't as much need to hunker down for weeks or months nowadays IMHO. And as Chernobyl and Fukushima demonstrate, it's not like these radionuclides that contaminate your area are going to just fizzle and disappear any time soon. The sooner you can leave the affected area, the less damage to you and your family. I think in your situation, I would worry more about the longterm damage to your family, and less on the acute effects so that's where my thinking is coming from.

So, assuming something like an isolated detonation and no major direct blast or gamma ray effects to really worry about, I think I would focus on evacuation. I may hunker down for a short time to let the most radioactive, short half-life radionuclides and fallout dissipate first and figure out which way the radioactive plume is heading, then proceed to bug out. Not sure if there would also be a mad rush to escape at first or not. Nutritional measures to mitigate radiation exposure inside our bodies, respiratory protection, particularly for the kids, and having enough uncontaminated food and water to make it to safety. Of course, if EMP is in play in your area, too, and knocks out all transporation... Hmm, that's a pickle.