Can you tell me
What kind of playing?
How old is the child?
girl or boy?
Are you the mother?
I gave your question a lot of thought....and although the answer is no, there are many possible reasons that this could have happened. First off, a man's erection or lack of, can depend on many things. A man can be with a beautiful woman, who is dying to be with him, and not be able to get an erection because he doesn't think he is good enough for her. A man can be around horses, and get an erection. The mind is a universe of untapped resources, and men will be the first to tell you, if they could pick and choose when they could or couldn't get an erection it would be a miracle.
Did you know that certain scents can cause a man to get an erection. Cinnamon and lavender are two scents which have been proven to give men erections. If a scent can give a man an erection, do you think its possible for him to control it? Some men get erections from the smell of baby powder, apple pie, wrestling and surprisingly tickling.
Did he know, that you're aware, that he got an erection? Did you talk about it at all? Was this the first time it happened?