The decoupage is an afternoon plus drying time overnight. The Kotel model is a longer project best spread across several sittings — it is the kind of thing to start well before Yom Tov and build a section at a time, which is part of the appeal.
Both work as family projects with an adult leading. Children take naturally to laying the mini bricks on the Kotel model, and the decoupage layering is forgiving of imperfect hands. The cutting and any sealing are the parts to keep for an adult.
Sealed, yes. A coat of mod podge or clear varnish over the finished decoupage makes it weather-resistant enough for a sheltered wall. The Kotel model is best placed on a table or shelf inside the sukkah rather than hung, and brought in if heavy rain is forecast.
That is the point of building something this substantial. Both store flat or boxed and come out every Sukkos — and the Kotel model in particular becomes the thing the children look for the moment the sukkah goes up.