I thought you had to add terps to it if it was gobby? Sounds a bit awful for kids ha ha.
I am not sure about the little oil paints in the paint by number. I don't remember using terpentine; but I remember they were hard to clean, and you could not just wash off the brushes like you do with waterpaints and acrylics, so it might have had a little jar of paint-thinner along with the kit.
When I used the little bottles of enamel paints for the figurines; I thinned that with nail polish remover. That might have been what I used for the paint-by-number ones, too.
Back in the 1950's; we didn't have all of the restrictions on what a child could or couldn't use. They expected our parents to have enough common sense to not let us hurt ourselves, and to teach kids how to use things.
Pretty much , that worked.
Now, they try to make everything safe enough that no one can hurt themself; so kids don't have to learn to be careful, like we did .
I am not sure that this is a good improvement....