I don’t think resin would work for this application. You would need a mold and it is stinky.
I think what you would want to use is polymer clay that you bake.
This site has a lot of information.

I would go to an art supply store or a hobby store. there are other options beside resin. You should check with those places. They have all kinds of materials to sculpt with. I’m sure they can help you find what you are looking for.

I assume you mean epoxy or polyurethane resins? The fluid casting stuff? While in theory you could work directly in cured resins I would not recommend it since in both cases the dust can affect your health. What’s more common is to make a master from another material, make a rubber mould from that and cast copies for sale or just because the copies are stronger. If you go to this site you can see examples of the mouldmaking and casting process:

I’ve never worked in newclay but for 75mm (which is three times the size that I work in) the most common materials are polymer clays like FIMO and epoxy putties like Aves, ProCreate and Kneadatite. Both have their own properties:
- Polymer clays: do not harden until baked, available in many colours, feels like a soft clay, cheaper, readily available at craft stores.
- Epoxy putty: hardens within an hour so work has to be done bit by bit, feel can be from clay to chewing gum - with some you can regulate this by changing the mixing ratios, monochrome, most are much stronger than Polymer clays, found at scale modelling and game stores. Many brands and qualities are available, not all suited for sculpting.

Most mini baby sculpts that I’ve seen are in Polymer clay.