What edible solution can I use cover biscuits to make it waterproof and stronger?

Can you tell us what the purpose of the biscuits is? If you just want hard biscuits for decoration, then do not use any fat or egg. Pretzels are hard but brittle because they have oil in them. However, I have found that using just water and flour, and maybe salt to tighten the gluten mixture, the resulting cooked dough is hard (I shallow fried thin pieces of Chinese dumpling wrapper dough, which has just water and flour, nothing else. If you are baking it, I won't be able to tell you what temperature and duration to bake it for). Kneading the dough will create a tougher dough and when you bake it, it will be very hard, but it will not taste very good. If you want it for or teething, it will not be good for babies because if they do break it, the biscuit might cause choking as it is quite tough to chew.

If you are looking to stop the surface from absorbing moisture, then the less porous it is, the better. A few ways you can do that is:

-Do not use egg, milk, butter or oil in your biscuits. Biscuits with these ingredients have rough crumbs and rough surfaces caused by bubbles in the biscuits - these bubbles/gaps lets in moisture and the crumbs absorb the moisture.

-coat the surface with an egg wash, this can create a thin barrier between the inside of the biscuit and the air

-coat it with some form of agar jelly - gelatin melts in heat so can melt if placed in water depending on the temperature. Agar is tougher and does not melt with low heat.

-Coat it in hard candy coating - the type you get on the outside of MnMs chocolate. It is not water proof but it will create a seal around your biscuit to stop it from absorbing moisture.

Experiment to see which combination works for your purpose.