I may have the wrong end of the stick here as your post is very difficult to read and understand due to the complete lack of full stops but...
Are you saying you were charged with murder which went down to occassioning bodily harm? If so, I assume her son died?
Either way, if your ex girlfriend told you to jump off a bridge, would you do it? No, you wouldn't. So why slap her son just because someone told you to. Anyway, back to the actual question.
I realise that you may have made a big mistake and could have changed in the 11 years since, but if my daughters boyfriend had been accused of murder or had been done for hitting children and I was a foster carer, then I would want to know. You gave away your rights when you got done for hitting the other child. If you truly are a reformed character and her mum likes and trusts you, you should have no problems.