Yes it is trespassing. The property owner, in this case the HOA, have clearly demonstrated that the retention pond is off limits (likely for safety and liability purposes) and so a person walking into the clearly posted trespassing area would then be trespassing. It doesn't matter if the boys have permission from the parents; the parents don't have the ability to permit the trespassing. The boys need to walk around the retention pond.
This is not to be considered legal advice nor does an attorney-client relationship exist.