Is it trespassing to be in a ditch?
My children love to play in the ditch by my neighbour's house because there is a small, very shallow creek that runs perpendicular to the road and his fence. They always stay outside of his fenced yard and play between the fence and the road. This neighbour has recently put up 'no trepassing' signs along his fence and recently asked some kids and their parents who were supervising to leave his ditch and heed his signs.
The answer to your question turns on the ownership of the land on which the ditch is situated. The fence which your neighbor has erected would seem to indicate that the property line ends there, and that the ditch is on a public right of way. If it is on a public right of way, then your children have every right to play there. If, however, the ditch sits on your neighbor's property and was dug pursuant to an easement to allow drainage, then it is his property and he has every right to limit access to it. In that instance, yes, your children would be trespassing. My suggestion would be that you consult the local tax maps, etc. to determine if the ditch is on a public right of way or private property--if it is that important to you to pursue this matter. Either way, no one is going to be happy with the outcome.
This may take some research on your part. Look at your deed. It may be the same or similar to your neighbor's deed. Does your roadside boundary go to the center of the street, to the edge of the street or some other point. Are you in a subdivision? Is there a subdivision map in your home paperwork? Are the streets and berm government property, i.e., municipal, state etc. or is it an association? Who or what entity mows the grass around the ditch?
You could go to the courthouse and look up your neighbor's deed. Depending where you live it is increasingly easy to find such information given our digital age. Your county may even have such information on line.
I would not rely on the fence as an indicator of ownership. Rights of way just render the property servient to the holder of the right of way. It doesn't mean that it is public property. Your neighbor may still bear liability for injury on the property if he is the owner. Zoning, covenants and restrictions may have limited the extent to which the neighbor could fence his property. On the other hand maybe a chocolate cake or some other generosity will soften up the neighbor.