It is important to communicate with children at an age-appropriate level. There are several cautions for doing so.
Imagine if you share "BPD" or other specific terms. It is almost a given that at some point that anyone - whether adult or child - might repeat it to the disordered parent. And you can guess how that would be received. And a child may not be able to weather the response.
Mental illness is hard for even us adults to comprehend. That sort of non-logical behavior just doesn't make common sense. So children will do better with examples familiar within their frame of reference.
Nearly two decades ago there was a booklet written that was written for minor children, using simple terms, examples and ways to deal with persons prone to periods of poor behavior. It is listed on our
Books board and the booklet is named
An Umbrella for Alex. It is well suited for a parent or therapist to cover and discuss the material. The story reassures affected children that they did not cause nor are responsible for a disordered parent’s volatile behavior.
There may be other resources for children but this is one I recall.