legal responsibility of the church

Depending on the circumstances, yes, the church could be held liable for injuries on the property. Since the church is now aware of boys riding dirt bikes on the property, they could be held to an even higher standard if someone was to get hurt on the property.  The church should take steps to prevent the boys from riding on the property.  To start, the church should post numerous "no trespassing" signs and also consider posting signs like "enter at your own risk", etc.  I also recommend you get some legal advice in your local community.   Your state may have state specific laws regarding this.  I am confident that the church can get some free legal advice from a local attorney in your area. Best of luck. NOTE:  This response is general in nature and should not be considered legal advice.  No attorney-client relationship exists or is formed by this response.