Explain that some fish are too small to keep, and that you may not keep them all. This could avoid great misunderstandings later. But do take something home for dinner, if possible. Prepare the kids for the event. Younger children may not realize that fishing means baiting hooks and enjoying the great outdoors.
Most kids enjoy a totally new event even more when they know what to expect.
Bring some snacks and drinks. Water or other drinks are especially important to avoid dehydration when you're out in the sunshine. It's not all about catching the biggest fish in the pond.