Hi and welcome aboard ! 🙂
Designing children is very easy, just use the pro version, export whatever you need and change the proportions of the adult figure to make him/her more childlike.
You can add as second hair color the same way. Once again, use the pro version and manually recolor a second copied hair layer. That’s very easy to do actually.
Besides these options, I’m pretty sure LBandy will tell you you’re dependent on the artists’ good will but that would be for him to answer 😉
My guess regarding the arms you’re referring to is that some layers are mutually exclusive, so, my answer would be “yes prolly”. But let’s wait for a more educated perspective… ^_^