I am a 21 yr old female. I occasionally slept with my parents when I was younger, not sure what age I stopped. If you have a normal, healthy relationship with your child/children I don't think it should be an issue. If it was an everyday thing, which you said it wasn't I think it would be a dependence/maturity issue more than an inappropriate issue. Honestly I wouldn't give a second thought to sleeping in the same bed as my mother or father. (except for that they snore horribly and it keeps me up).
I agree with your therapist, if neither of you is uncomfortable, there is no reason for it to be wrong.
I think that other relatives/non relatives is a separate issue. But, parents? You are supposed to be close to your kids. If they are ill, you should be there as a support. I think that jumping to the conclusion that it is inappropriate bc of the gender difference is a little oversensitive. Just my opinion though...