I don't believe there is a solid answer when dealing with children, because they do understand more than we think, though they just display things differently. Nobody wants their five year old thinking they have to take care of mummy, otherwise they growup with their own little unique trauma in adulthood.

I think honesty is the way, though really you even have to limit that or put it a way a five year old will understand. That I guess is the challenge in itself though! Firstly you cannot feel guilty for helping yourself, because if you don't do it now then your child will suffer later from your illness. Get it over with, get it done, so you can focus back to being a mother and not a mother with PTSD overwhelming her constantly.

I would ask your therapist for specific advice maybe surrounding the best way to approach your five year old about this....