I think one small reference in an amusing story is a gentle way to break the reality of the situation in a little, without being harsh
I find myself wanting to let them know about the difficulties my DC are having as it is a real picture of the person they are becoming
I very much agree with both of those comments, the second really strikes a chord with me because that is what I wanted to achieve in my letters to DD2 and DS birth mum (I don't write any more, because DS doesn't want me to, and DD2 facebook chats with her birth mum a lot and has met her a lot this year, so we're well out of letterbox stage now)
I wanted to make the kids real, good bits and the harder bits. I was always certain that DD2 would get back in more regular contact as an older teenager/adult, so I wanted to give her birth mum a real picture of her, as a lovely girl who has achieved x,y,z but also a girl who does have emotional/behavioural/mental health problems, because I didn't think it would be helpful to only include the 'good bits' and then have birth mum totally blindsided by the girl/adult she would end up meeting
Of course, some birth parents will simply live in denial, and will not ever accept the reality of the situation, even if you do write the word 'mummy' or say that your child has difficulties with a,b,c. But we aren't responsible for that
I know some people write a few sentences and nothing else, and whilst i really understand how difficult letterbox can be, I think it's pointless to only write one paragraph and might be more hurtful than helpful, to everyone. I can't help thinking...force yourself to write a bit more, or if you really can't do that, then why are you doing letterbox at all?