Some things are easier than others. Are you thinking about the animals as well? It would be easier to create those in a program like Blender, if you go onto youtube and type in something like “Modelling for 3D print” you can probably get a few videos to help you. As well as “Subd Modelling in Blender” or “Sculpting in Blender”. My biggest tip would be to make sure things are fairly chunky and bevels are a bit larger. You can probably look into “Miniature Sculpting” either for Blender or Zbrush, the principles apply to both software. I only suggest Blender because it’s free and a decent sculpting application.
For the Castle and what not, you can use Shapr, if you’re wondering what kind of rules for 3d printing you can probably apply them from other Cad software and I think the Shapr Youtube channel has something on 3D printing as well.
So hopefully that helps you with getting started, I’d say the animals will be the most time consuming part if you want them to be like sculptures ( which I think would be better than billboards since your child will probably want to pick them up and hold them).
So best of luck!