I make gingerbread houses every year. Walls should be approximately 1/4 inch thick. I have found that cutting the pieces first to work best. Yes there is some spread, but when they come out of the oven, I replace the stencil and trim up any spread (for the straight outside edges, a pizza cutter works well). I know it sounds redundant, but sometimes I don't cut quick enough, or I am distracted by the phone, or any other number of things, if I only cut after baking. With the items pre-cut, if I get distracted, I at least have a usable part. Granted I may have to shave any spread very gently with a microplane or the edge of a knife, but that is just what works for me.
I also will roll the dough out on parchment paper, cut and remove the excess, then slide the whole thing onto a pan for baking. This way, you don't get any "stretch" of your pieces. I have several square cookie cutters that I will use to cut out the windows and such. I make the windows rectangular by using the cookie cutter and cutting a square then moving it and cutting another half square. This is helpful because when the gingerbread comes out of the oven, you don't have to spend a lot of time with a knife trimming the inside of all the windows... you just punch any spread out with the cookie cutter.
As Sobachatina mentioned, melted sugar makes beautiful windows. I however, will lay the walls on a silicone mat and pour the sugar directly into the window holes. No need to "glue."
I'd hate for you to make a gingerbread house with the "wrong" icing. You want an icing that will dry rock hard, like cement. For this you will want to use "Royal Icing."
For larger houses, use canned goods to hold the walls in place while the icing dries. The house should be built at least one day before you start decorating, to be sure it has set up really good.
All of those ingredients should be fine. The only thing you really have to worry about is shelf life, so make sure to buy fresh ingredients and check your expiration dates, especially on the baking powder and baking soda and the almonds. I would just tell them to use it within a year (or the earliest expiration date).
Oh yeah, and don't put the baking powder and baking soda next to the brown sugar. There isn't a lot of water in it, but it might be enough to cause issue.
Best Answer
It will probably not affect taste, however, it will affect texture. The reason is that the flour will react to the moisture in the batter and start to convert to gluten. This will produce at best a chewier bar (which may actually be desirable) or, at worst, a rock hard bar.
If you are willing to take this textural risk, then I would recommend putting the pan directly into the oven from the refrigerator. This is because the bars will be so thin (and thereby have relatively large surface area) that they will come up to temperature in the oven quite quickly. You may have to cook them slightly longer; just make sure to test them with a toothpick or, better yet, a thermometer before removing. The internal temperature should be a little over 200°F or a little under 100°C.
If you want to minimize the risk of chewiness/hardness, do as little initial mixing of the flour with the batter as possible. It is okay to have some small lumps of flour since they will absorb the batter during the resting time in the refrigerator. This suggestion is due to the fact that gluten is formed both by moisture and mechanical agitation (mixing).
Another option, for which there is no potential harm, would be to mix the wet ingredients in advance and refrigerate them, also mix the dry ingredients in advance and leave them at room temperature, and then mix the wet with the dry just before putting everything into the oven.