Is a parent leaving internal or external conflict

It can be both.

Explanation:

It hasn't happened to me, but I suppose it would a be a painful experience. It can be internal because the character might have turmoil with their feelings and how to adjust to the situation. This especially if they are a child who depend on their parents emotionally, financially, etc. In almost all aspects. The character might struggle wih themselves to understand what happened and why it happened. They can maybe blame themselves and get into self-conflict with themselves that turns into self hatred.

It can be a external problems if the child argues with their parent about this. Why they're leaving, for example. It depends on the authors estabishment of the relation between the child and parent. For example if the parent hates the child, they might shout at them and they might argue (remember external conflict is any conflict between the character and something/someone other than themselves)