Nothing. But I would certainly call CPS and let them know that child came over and BOTH mom and step-dad were unaware of him leaving.
As far as keeping DCB's "secret" goes, I wouldn't feel as though it is my place to get involved.
Mom will find out soon enough and bio-dad is more than likely just talking smack to his kid and the whole gaining full custody thing will NOT happen.
He may try but I doubt he will get very far considering what kind of parent involves their 10 year old in something of that nature
I would encourage DCB to talk to his mom though about anything in his life that makes him sad or unhappy (including his step-dad) but I would walk carefully when it comes to getting in the middle of anything else going on.
I know we have insight in to these kinds of things all the time but IMPO, it is still not part of the job description to get involved.
I am sure others will think/feel differently but in the end only YOU know the right thing to do based on your relationship with DCB and DCM....kwim?
Some providers are VERY personal and some are not.
I am not. I concern myself with the immediate day to day care of the child and that is all.