Reply by Charlie

Milk paint is actually sacrificial by nature. It will slough with weathering. You can make it FAR more durable by adding NO MORE THAN 10% pure tung oil or real boiled linseed oil. NO MORE THAN 10%. I would only add the oil for furniture or outdoor items to slow the weathering effects outside or make a furniture piece a little better able to withstand a scrubbing.
For kids’ toys it’s just not necessary. Milk paint is a flat finish. No shine. You might get it to an eggshell if the piece is sanded very finely, but in general, milk paint is flat flat flat. For that reason the childrens’ toys might be more attractive with a shellac overcoat.
As an artist I’ve worked with casein paints for over 40 years. So I kinda know how they work. :)