Basically what the Baron said, Weekend edition kits are just the basic kit, the standard kits may include a small amount of PE or resin, the Profi pack kits include the most popular aftermarket bits Eduard makes for the kit and their Royale kits include all the aftermarket bits Eduard makes for the kit.
The Weekend kits are still very nice, and well detailed, they don't take any short cuts assuming the plastic will be replaced by PE or resin.
They also offer quite a few dual kits which can be a real bargain.
As far as WW1 kits, I build 1/72 but I think the quality carries over the scales. For a kids bedroom I'm assuming 1/72 or 1/48.
Eduard, Roden and Revell have very nice kits. The Roden kits often include parts to build different versions from one kit so can be a bit fiddlier (cutting and replacing parts), but they also include large decal sheets some with 4 or 5 different sets of markings. The Revell kits are probably the best bargain, being cheap but still nice kits and very easy to build. Airfix offers some unusual subjects, the kits are pretty basic but go together fairly well and look nice on the shelf or hanging from a ceiling.
If you are considering 1/32 Wingnut Wings is a new company getting very good reviews. Eduard and Roden offer kits in 1/72, 1/48 and 1/32, Revell offers 1/72, 1/48 and 1/28.