Hello and thank you for the opportunity to assist you. My name is ***** ***** I will do my very best to answer your legal questions.
If the motorcycle was given to your husband before the friend passed away, then the motorcycle belongs to your husband, and the will is irrelevant. Even if the will had the motorcycle in it and purported to give it to the children, they'd be out of luck. The will can only give property that is owned by the friend's estate. It can't give property owned by other people.
However, you mentioned that the executor hasn't given title to your husband. That leads me to believe that the motorcycle wasn't actually given to your husband prior to your friend's death. He may or may not have given the keys to your husband, but that's not the same as giving ownership.
As for contesting the will, yes the children may contest its validity, or they may contest whether or not the motorcycle is actually part of the estate to give to the children.
Does that answer your question(s)? Please let me know if you need clarification, as I am happy to continue helping you until you are satisfied. Also, please remember to provide a positive rating via the stars (and note that your positive rating is the only way that I'll get credit for helping you, so it is much appreciated!). Thank you. :)